Hi Martyn!
Aha, well the good news is you don't need an LPG garage!
Easy one first - try some fuel system cleaner in the petrol tank.
Many products are available, but I have used Cataclean in the past. If you go with that one the full bottle is poured in with around a quarter of a tank, don't put it in with more, it can't do the job properly. Then take it for a good drive.
Another is Millers Petrol Power Ecomax, I use that for the octane boost property, in the Vellfire and our Subaru ( it's the 3.0 H6 so likes high octane too like these 3.0s and 3.5s).
That's a 500ml bottle but it will treat up to 500 litres. Or much higher levels for a clean up.
Millers do a diesel treatment too, 500ml again, and our local diesel injection specialist company said it works when I took my previous van in to them, they saw it in the van. It makes their job easier apparently, and they can tell the difference when they strip injectors.
It wasn't sales patter, they didn't sell it!!
That was 20 years ago, and I've been using it since.
Many will say they are "snake oil", but I reckon it does work, by no scientific research just "feel" of the engine and "seat of the pants" driving. Doesn't feel quite as smooth if I've run without it for a few fill ups, usually 10 gallons. Not tank fulls, the van takes 100 litres!!!
PLEASE come back and let us know how you get on! If you get a good result then it's information for the whole forum!