Can't help with servicing in North London, but heading south a bit, Ian Ellis is very good in Brighton, he posts a lot on the Owners Club Forum and certainly knows his stuff.
Source for parts is easier, in no particular order;
Highwood motor company, Swansea, Chris Sweetapple. Very quick service, and great guy, good website too.
EB Spares, Kevin. very friendly been in business longer than anybody else.
Alfaholics, The Banks Family, best for performance bits, but very good for other stuff too, another good website.
Classic Alfa, Richard Norris, more local if you want to go and visit, excellent website.
All the above would be found by a google search, they are the four main suppliers of new parts in the UK.
For used parts just keep browsing Ebay, there are various regulars who always have lots of 105 bits.
Good luck with the Spider
