Try also Brian Cook of Bournemouth Avionics (based at Goodwood).
He installed a Garmin GTX328 and a GMA340 in our Warrior last December and was the best quote of the thirteen I sought. His work was fine and he delivered on time. No complaints at all.
PM me for contact details if you're interested.
I sought quotes from all of the companies so far mentioned and they were all good enough to respond with proper, written quotes.