STAG is the leader on PL market and very popular in the whole Europe.
It's more about the installer instead of the system I believe, a great system fitted just to fit will be problematic.
From what I can see there is no Stag Approved/uklpg installer in Deeside
http://autogas-network.co.uk/, https://www.drivelpg.co.uk/
You may want to double check this to avoid issues with insurance and other problems.
Your price seems to be similar to other installers of STAG, however there are few "recommended"(on forums) charging in the range of £1200-£1500 for the same system. FYI to install this system in Europe to 2.4, 4 cylinder car it's no more than £500-£600 (In the country where LPG is a norm as a dual fuel).
Hopefully someone will provide you a "genuine" response to your question about own experience with the STAG.