Foffer said:

Are there any regs regarding the compatibility of all BS1363 sockets and plugs, if not should there be?

Yes, the Plugs and Sockets (Safety) Regulations 1994. These require, for products on the UK market:

1. Plugs (but not adaptors or "charger plugs" etc.) to be independently certified by a notified body; and
2. Socket-outlets and adaptors for use with BS 1363 socket-outlets to comply with the relevant part of BS 1363.

I guess the issue being, that the "adaptor-like" plug won't mate properly with the "socket-outlet" because the standard stops at the relevant interface points (or a little outside them), rather than considering the dimensions of the "enclosure" of the outdoor socket-outlet?

So, for example, this particular product, which has (as far as we can see) a dimensionally-conformant which standard socket-outlet: https://www.screwfix.com/p/masterplug-ip66-13a-1-gang-weatherproof-outdoor-unswitched-active-plug-socket-with-rcbo/889rg

Would accept a "standard" (moulded or rewireable) plug, but perhaps not close with the plug fitted to this extension lead by the same manufacturer?  https://www.screwfix.com/p/masterplug-10a-4-gang-25m-cable-reel-rcd-240v/55494 ?

Basically, the standards don't extend to what's behind the plug, and also the outdoor enclosure in which the socket-outlet is mounted.

There are unintended consequences if we decide to regulate the dimensions of the rear of the plug, for example it would result in a "ban" of the "easy-grip" plugs used by people who don't have as much strength in their arms, or limited mobility or use of their hands): https://cpc.farnell.com/pro-elec/pe01012/plug-13a-mains-easy-pull/dp/PL13411

You could use a weather proof extension lead such as www.argos.co.uk/.../9094653

b)The electrical equipment, together with its component parts, must be made in such a way as to ensure that it can be safely and properly assembled and connected.

Above from Schedule 1 of the Electrical Equipment (Safety) Regulations 

If you do not get a satisfactory response from the manufacturer, I would recommend contacting EH Department of your local council.

A neat solution to power supplies transformers built into plugs  and so on can be the  drybox approach

Example here.

https://www.festive-lights.com/dribox-weatherproof-connection-box-black-and-green

At work we have used milspec versions by performing the same tricks with pelicases, though I suspect that will cost more than the BBQ.

I do quite often find myself thinking that we need to permit/encourage a more outdoorsy plug in the UK for domestic garden use. The square pin is all very well in the dry, but the mating plane has no water break. Perhaps a version of the old BS4343 with earth pin operated shutters would be worth inventing.

Mike.

Thank you for your reply, I appreciate your comments around the easy-grip plug.  I also note the next comment by OlympusMons who suggests using one of those big weatherproof socket boxes which I suppose would do the job.  It would have been nice if this was suggested in the manual for my BBQ!

Thankyou, you are indeed correct, I could use one of them.  I'll suggest to the manufacturer that they include this advice in their manual

Thank you, I'll wait for a little while, then do this.

An aside,

This idea that all warranty /guarantee is cancelled if you take the plug off is not generally correct, and in terms of statutory rights as a buyer of an appliance, is not enforceable. What you cannot of of course claim for is if you get a shock because you mis-wire the replacement plug, or if somehow you damage the cable, but there are very few ways you can do that, where the house RCD would not save you. Your normal consumer rights in respect of the rest of the machine remain.

However, what may be at risk are extended cover and protections that the makers offer as add-ons ,where of course they can insist on whatever odd conditions they fancy to get out of any claim made on it  they like - you return it in the original box,  you get it serviced annually by their agents or what have you.

However the more responsible suppliers acknowledge that you may need to change the plug and say that you should only do so in a certain way.
It is not soo long ago that  we were fitting round pin sockets, and there are some still in service, though not many these days. For this reason, until quite recently, regulations required that mains leads came with a little sticker telling you the colours off the wires within in case yo had to change the plug - unless you are taking the plug off, the internal colours really could be anything (but really should not be).
Mike.

Thank you for your suggestion about the box, this would indeed work, and your valued advice about whether or not to change the plug.

I haven’t tried one of these yet but saw it and thought it should work for just the situation you described.

https://www.tlc-direct.co.uk/Products/BGWP23L.html