Hi Gavin,
The nearest place I can think of is https://www.mcgrathmaserati.co.uk I know they are about 90 mins away but they are well respected.
If you work in London there are a few places that you could drop the car off and pickup at the end of the day e.g. www.verdimaserati.co.uk West London. used Karl for service and sales.
https://autofficina.co.uk, however South London so a bit further, used Elias for diagnostics.
https://www.prestigeautoworks.co.uk/maserati-servicing Uxbridge, used Dale for service and preventative maintenance.
These might be closer to you though.
http://www.qvlondon.co.uk/ QV London. Unit A1., Fairacres Industrial Estate, Dedworth Road, Windsor. SL4 4LE
http://www.bhpmsport.com/ Near: Aylesbury, Bucks
Let me know who you choose & best wishes
Tony