Thanks, its the blindhog method. Google it and you will find the info but it's basically a caustic soda and potassium nitrate mix which seems pretty easy to make up but can be quite dangerous if you have not worked with nasty chemicals before!
I have another recipe to try now so will get some chemicals ordered this week and try those next weekend as long as its not chucking it down.

Its a expensive setup but if like me you have quite a few guns that really need doing then it works out quite cheap overall.
Cannot wait to get the heavy oil off and see how its come out.