Have to agree with MrFuzzy there they are well known for robbing people in the trade as well as public! Edmondson seem reasonable most of the time or kew if they have one round there!
search the cheapest delivered price on line. I think it must fluctuate because sometimes the factors cant get close other times they can better it. Don't bother with City normally only use them if nobody else has it and want it fast, the sparks I know all say the same. Funny thing cable though sometimes someone has what you need in stock wants to do a deal - shop it widely on line first though as even contractors sell off unwanted stock at the end of a job.
u tend to strike lucky with heavy cables at autojumbles 50 meter coils or 100 as most punters dont need that length and the sellers cat get rid so u make em a daft bid and buy cheap