Total Driving, based in East Anglia.
Prices
6 Hours (1 day) – £1112 (Suitable for LGV/PCV driver)
10 Hours (2 days) – £1812 (Suitable for non LGV/PCV driver)
Prices include the practical test fee of £62.
https://www.totaldriving.net/other/h-tracked-vehicle-training/
Looking at their prices, Tanks-Alot's 2 day £1,200+VAT course doesn't look so expensive.
For help with a G licence, you could try a local steam club or rally. I'm not suggesting that you take a test on a steam roller but I have seen diesel rollers at shows.