See for £100 you could get yourself an Aldi compressor, or even get one from machine mart for a little more (then its yours for the next job) machine mart also do the guns for £10 and the waxoyle for £5.
The gun comes with a length of tube.
Jetwash the chassis (if you don't have one the garage ones are powerful go there and use that - you might get some funny looks crawling about under your landy) then get under and wirebrush any flakey rusty bits. Grind/sand any bad rust bits right down. Get into all the dodgey areas and scrape out any muck.
Let it dry for a day (if your getting good weather at the moment all the better), then get in there and blast waxoyle all over it, i rekcon for less than £200 you could buy a compressor, about £25 worth of waxoyle and a £10 gun.
So its going to cost you £100 more this time, but next job you will have more tools.
Also I am not so sure these places that do waxoyle are really going to care about your landy rusting, your going to spend a lot more time on it, and your going to bother with all the little bits. If someone gave me £100 to do to there chassis what I did to mine I would laugh.
Right I am off to see CharlesY.