Buy the panels yourself, from Wickes, for that £100 (plus £30 delivery if you're within 10 miles of a store, £35 up to 20 miles, £45 up to 45 miles):
http://www.wickes.co.uk/
Then seek out a general handyman/builder/gardener (rather than a fencing company) to install them. Even with a two-man team demanding £150 each for the day's work involved, you still be saving at least £300.
Fencing can be an expensive job. Are you having concrete posts and gravel boards? The old fence taken away? It all adds up. Get as many quotes as you can, ask around to see if anyone can recommend someone trustworthy.
No way would this cost that much. We were once quoted for a fence across our field,post and wire at £13 a meter - the next two guys that came to give a quote said between £1.50 and £1.75 -I even rang the first quote to see if it was a typo but no -some people just see a posh house, or some other indicator you can pay and try their luck.
Base tradesmen quotes on around £25 labour (outside London) and that job sounds like a short days job for two men. 10 hours at very most = £250
Definitely get other quotes. Some contractors, if they don't want to do a particular job often quote a ridiculous price just to ensure they don't get it. Seems a strange way to do business but there's nowt as queer as folk.
Five fence panels, six posts, plus concrete mix (maybe kick boards) for £100.00
I take it we are not talking six foot wooden fence panels here? was the quote over the phone or an on site estimate?
You say £100 for materials .. are you looking at a 1994 catalogue ... that sounds realy cheap to me. If the fence is in need of 'Total Replacement' as stated. Where do you manage to get 5 panels, 6 posts, and 6 bags of postcrete and 30 panel fixings for £100. If the old post bases need to be cut out with a kango then the hire fee will be on top of the price. Not to mention removing the waste and disposing of it.
All of a sudden there seems to be a great big hole in that £650 profit you imagine.