As your car is outside the (normal) 3 year warranty you would be best off going to an independent who knows about the 350's and as has been suggested, you will generally get a better job done and not at the OTT dealer prices.
I know you have asked about a garage near Twickenham, but a relatively short trip down the M3 to junc 3 will find you TGM sport at Fleet - see:
http://www.tgmsport.co.uk/
Ask for Tom - he knows the 350's very well and although their reputation was built around racing Hondas I, and numerous other members here, have found they do a really quality job at reasonable prices.