Hi there,
not too many specialist places in London that i've found. I've used volkspares before, they seem good and friendly from my experience.
Hopefully thats not the kiss of death !
heard many a horror story of volkspares, for parts maybe, the bad strories have outweighed the good esp when it comes to reconned engines. I think jacks garage is out that way, not used it my self and I think hes mainly Aircooled, but doeas water too, http://www.jacksgarage.co.uk/ someone on here will know more about him.