Diamond tyre warmers from ebay £130
Generator hire for the whole weekend £35
Those little two stoke generators for £70 wont run both warmers together (the warmers are typically rated at 600 watts each) so you would either need two gennys at £140 or get a four stroke 2kw genny for about £200. All prices are ebay style ones but come up very regularly so it shouldnt be a problem.