An impact wrench is the sort of item that tyre shops use to remove wheel nuts.
Impact Wrench
The item in the link above screws on to your locknuts by means of a left hand thread and as it tightens onto the top of the locknut it loostens it from the wheel. I would have a word with your local garage and see if they can help, if not a local Kwik Fit should have the correct tools for the job.
It might be worth going to a scrapyard before hand and getting 4 plain nuts (to replace the locking nuts) before trying to remove the locknuts.
You say that the Rover garage sent you a security nut remover, what is it like? did you use it properly?