Thanks very much for all the excellent advice folks,it's very much appreciated.I am waiting for the site manager to get back to me regarding how much the previous cleaners quoted.I think this will give me a good idea as to whether the job will be practical for me or not.I think if he comes back with £500 or less I will probably give it a swerve as I can't justify taking the time off from my usual work.
Thanks to the people who have offered their services,can't really take you up on it till a price is sorted but will bear it in mind.