The super capacitor on my wireless Davis VP2 has failed. This is the second one I've had, the first replacement I got from the UK weather shop but that has now changed owners.

Can anyone recommend where to buy a new one from?

I have used http://www.weatherstations.co.uk/  for my VP2 spares for several years now, used to be called prodata

It's nothing special, you can get it at Maplins for about £5 delivered - but fitting it is not simple, and it could be that you have another component which has failed and the drain will continue.
That's what happened to mine, on balance I'd advise you to buy a new board and then look into repairs to the old one so you have a spare ready to plop straight back in.
This is a good deal on them.
http://weatherspares.com/epages/eshop1036725.sf/en_GB/?ObjectPath=/Shops/eshop1036725/Products/DAVISS

Any use?

http://www.ebay.co.uk/itm/Davis-Vantage-Pro2-Wireless-ISS-Transmitter-Board-Super-Capacitor-LOW-STOCK/191251738887?_trksid=p2047675.c100011.m1850&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D20140107083358%26meid%3Dbf1b72be9d8341f6a9a24c7fb662edbc%26pid%3D100011%26prg%3D20140107083358%26rk%3D1%26rkt%3D10%26sd%3D191182266859

That's what's fitted - they use the 10F one.
If fitting it yourself it might be best to solder onto the legs of the old one rather than try to plug into the board.
I did do that successfully but not being a frequent solderer, it was not easy - and could soon have gone horribly wrong.

It's nothing special, you can get it at Maplins for about £5 delivered - but fitting it is not simple, and it could be that you have another component which has failed and the drain will continue.
That's what happened to mine, on balance I'd advise you to buy a new board and then look into repairs to the old one so you have a spare ready to plop straight back in.
This is a good deal on them.
http://weatherspares.com/epages/eshop1036725.sf/en_GB/?ObjectPath=/Shops/eshop1036725/Products/DAVISS

I've replaced one before and the fitting was quite straightforward. That worked for a while but then another component failed and I replaced the ISS board about 18 months ago. TBH having a supercap fail on an 18 month old ISS board is disappointing.