Ideas that may not apply -I have not seen it, but could you bury a duct, at whatever  depth you can sensibly get in over the rock, and leave it empty or just with a draw-line when it is not Christmas? 
Pull whatever cable you need through  for the duration of the event, and have the controls as near as poss to the unmetered supply, so it all goes dead when switched off. this ensures that at the worst the summer fayre tent  pegs hit an empty plastic tube....

Our town's Xmas lights have a control box that padlocks into a thing in the ground looks like a parking bollard, but is then unbolted and taken away for  the rest of the year to a dry place free of folk who might do something silly to it at closing time.... all that remains visible is a plate about the size of a hydrant  cover, but not with the same markings. Ensures it  is not broken when the time comes to do it again.

Mike.

Edits,

Apparently it really used to be a parking bollard of the removable kind, and then control box was then clamped to it  and a slot made for cables to be run down inside it so the wiring is not exposed until you uproot the thing, but since the town square has been redone with fancy paving there is now a convenient new power box that is used for other stuff as well. https://www.woodleynet.co.uk/xmas.htm shows the older version

The twinkling ELV designs can be 'cut and shut' extended with multicore cable  within reason. However our one still had mains on the tree and the overhead festoons on was when I was last involved in any way - though to be fair that was before lock down.

Most ELV Christmas lights are rather dim IME. I would if possible use mains voltage festoon with 1 watt LED lamps.  On a 0.1 Kva Supply a hundred such lamps may be used. If actual illumination is desired, then consider 4 watt lamps, two dozen of these can be used.

An RCD is almost certainly required, and is prudent even if not specifically required.

Agree to install a duct and remove everything when not used.

Check carefully what the bill will be. Un metered supplies are sometimes charged for as street lights are, presuming all night operation for about 4,000 hours a year. Rather expensive if only used for a month at Christmas.

A weatherproof and RCD protected supply from a nearby building might be more economical, add a check meter to avoid doubt or disputes over how much energy has been used. A  13/15/16 amp  supply from a neighbour could be most useful for other outdoor events. The Coronation weekend for example.

Another advantage of mains voltage festoon with LED lamps is that different lamps may be fitted according to the occasion. Suggest red, white and blue for the  Coronation and red, green, yellow blue and white for Christmas.

How do you get several hundred 1 watt lamps on a 0.1 kVA supply?  Shouldn't it be no more than 100?

But a 20 mA LED at 3 V is only 0.06 W.  You could have vast numbers of those.

Yes you are of course correct, my post contained typos which are now corrected.

Many thanks for your suggestions. I had thought about a duct but had never thought about removing the cables when not in use which is an excellent solution. We did consider using nearby houses but that would require digging up across a sunken sandstone bedrock access road, garden walls and  the green which was rejected by the PC. As it is SPEN will have to dig up about 9 meters of green to get to a position where the box will not be too obtrusive, which I am sure will involve sandstone base. 

Using an old bollard base is an excellent idea but will be unnecessary if we take the cables out. 

The lights do not have to be too bright and ELV will be brighter than the feeble battery lights we have used in the past. As to having twinkles or always on I will have to consult with the PC. Taking out the cables when not in use means I do not have to worry about tamperproof and waterproof plugs and sockets. 

I am inclined to go for the ELV lights and put the  power in the supply box as the tree is easily accessible and while the risks with 230V may be low I think the location and the accessibility of the tree make ELV more attractive and nearly risk free. 

Again many thanks for your ideas. 

Kevin 

Another option might be to avoid mains power entirely and use a 12 volt battery instead. Cheap in capital cost and no disruptive digging.

Many vendors offer battery operated lights, powered by three AA cells. Remove and discard the battery holders and wire three such sets in series. Each set of lights will include a dropper resistor, and all three of these MUST BE RETAINED in the new series circuit.

ten such series string of lights will only use about one amp at 12 volts. A 12 volt leisure battery will supply that for many nights. 12 volt photocells are available for nightime only operation. A small fuse such as 2 amps is important.

An arrangement I once saw consisted of a local resident running a chain of domestic extension leads through an upstairs window of their house, up a front garden tree, across the public highway, to a tree on the village green, down to the grass, then running along the grass to power various lights for an evening event. The plugs and sockets were made IP68 by the cunning technique of wrapping them in carrier bags. (I am not endorsing this approach.)

The plugs and sockets were made IP68 by the cunning technique of wrapping them in carrier bags. 
well, at least it meets the letter of the plug and socket regulations. The use of waterproof connectors to EN 60309  probably would have been safer but apparently in contravention of the rules.  But joking aside, we've surely all seen this sort of thing haven't we, and while it looks pretty dire (*), in practice there simply are not enough accidents to  make it worth getting too  steamed up about..
Mike.

(*) in another role I have one talks of 'power of opportunity' when extracting electrons from somewhere not originally intended, and that often looks even more dire...