Twiggers certainly have a good reputation and I would be happy to take my Guzzi there for work.
I have created a list of Guzzi mechanics but there’s not that many in your area.
Another possible would be Gavin’s bike Hospital in Oxford. His website doesn’t seem to be working tonight, not sure if it’s a temporary blip, he does have a page on Facebook www.gavinsbikehospital.co.uk
Nigel at NBS near Stafford comes highly recommended. Another website not working this evening www.motorcycleservicing.co.uk