I regularly attend meditation sessions with the Samatha Trust in London, and I have spent a weekend at their centre in Wales.
They also have an active centre in Manchester, I understand.
The Samatha Trust is mainly concerned with the teaching and practice of Samatha meditation (no surprises there, then
) To that end they don't say much about Buddhism to begin with, they just teach calm abiding meditation in a secular way.
The group that I attend includes chanting, metta practice and some sutta study. Several of our members have been regularly attending for up to ten years. The teachers are all long-term experienced meditators, and they are always ready to help our practice with one-to-one discussion.
It's all Therevada-based, as the Samatha Trust was established in the UK by a Thai Buddhist ex-monk who travelled to London in the 1950's, and taught mediation in London and Oxford.
In summary, in response to your question "Are they any good?", I would say "Yes, without a doubt".
Of course, it depends what you are looking for. If you are seeking Vipassana or instant enlightenment or yogic flying, you are sure to be disappointed. If you are looking for a sincere and open group for developing meditation practice, try them.
Hope this helps... I would be very interested to hear how you get on, and whether you agree my assessment, or not.