Hi Stewart and welcome to the Club!
I would have suggested McGrath Maserati as they are near(ish) you just outside of Hitchin, but they currently only service up to the Quattroporte V, although it may be still worth giving them a ring.
If you fancy a drive, then take it down to Emblem Sports Cars in Poole. They do a Saturday morning clinic, and will put your car up on the ramp and give it a once over. Many in the Club use them and would recommend them.
Another option the other side of London is Giallo Cars in Tonbridge, again, well known specialists.