Well London's a bit poor for service engineers these days. Phil Delahaye moved to Brazil but he did recommend to me a guy called John Bright for Roland stuff (in fact he had my Jupiter 4 not so long back). He's in the Midlands though so you'd need a car or courier you trust. Also he's quite busy so turnaround may not be that quick. Tel: 01584 810 467
Do not, I repeat, do not use The Synthesiser Service Centre or Cimple Solutions in London.
If John Bright's not an option then there's EMS Southend:
and David Wiseman in Northampton:
I have not used either of these myself, so I have no idea how good they are. The faults don't seem that serious to me so maybe any half-decent engineer could do it - you might even try cleaning the keys yourself. If you do get it serviced properly then do yourself a favour and get the osc boards calibrated while it's open.