Outside light and timer, switch required?

Regs do not insist on another switch, as you can switch off at the fuse board if required, but if it is fed from a circuit supplying some or all of the rest of the building,  then if say water gets in, the whole lot could go off and stay off, which could be awkward. 

There is a lot to be said for the convenience of being able to isolate in such a case, but it is not a requirement.

I presume the timer can be set to an always off state, but that may or may not be desirable.

I presume you have already checked that there is a neutral at the existing light switch you intend to tap into - as quite often there are only 2 wires, a steady live and a switched live to the light fitting, neutral only being needed at the light fitting.

Mike.

I would consider a double pole switch upstream of the timer to be good practice for the reason already given of allowing continued use of the rest of the installation when the water gets in and trips the RCD.

Also simplifies replacement of the light or the time switch.

I have been known to supply outdoor lights from a 13 amp socket installing one for the purpose if need be. This permits use of a cheap plug in time switch and also user replacement of same.

MY outside lights are all connected through a round pin plug and socket.  That allows total isolation and allows the lamp to be tested independently  of the control circuits.
And probably is non compliant

dcbwhaley: 
MY outside lights are all connected through a round pin plug and socket.  That allows total isolation and allows the lamp to be tested independently  of the control circuits.
And probably is non compliant

That depends upon the location and type of plug and socket. A plug and socket outlet are a perfectly good means of isolation - Table 537.4.

A plug and socket outlet are a perfectly good means of isolation - Table 537.4.

Indeed .. unlike a normal lightswitch - which as well as not being suitable for isolation, these days isn't suitable for ‘switching off for mechanical maintenance’ (e.g. changing the lamp) either (as 537.3.2.1, perhaps somewhat bizarrely, now calls up the requirements for full isolation).