I've used Tamer Trailer Centre in Plympton (Vally Road). Tim (MD) or Ian are very helpful and well priced.
I had a Witter bar fitted on an older 2004 407sw which cost £350 using the proper wiring loom plug rather than splicing in, which invalidates the car warranty
They are on 01752 341133 give them a ring.
The other place I know in Plymouth is "Plymouth Trailer Centre" on Richmond Walk, Stonehouse. Not used in a long while though.
Hope that helps
Peter ~ Boatsandoutboards4sale ~ firstname.lastname@example.org
~ 07930 421007