Thanks for the replies.
Well, I went ahead and did it anyway, after carrying out more research and having a chat with the guy, who says all the right things.
It's a 3-man firm, he says that they cover a large geographical area and because of this, don't publish the address as they feel that this would restrict business from the more distant customers.
On the recommendation of John F, I also looked at another firm, seemed at first sight to be more professional but their website has a lot of knocking copy, which I personally don't like, plus typos, and I couldn't see any justification for paying twice as much.
The claimed improvement is 30 HP (from 115 to 145) and 80NM of torque (from 240 to 320). These increases are within the capacity of the engine and well within the capacity of the gearboxes, transmission and clutch, so should be fine as long as I don't drive stupidly. For me, it's about grunt, not speed.
Those who know me will understand, small farm in N. Yorkshire with very hilly roads, often towing heavy trailers.
I'm going to be on the farm next week, so will find out whether there's a real improvement or not, and will report back in detail in a few months, or if anything goes wrong.