Hi NJA
With regards to overcharging you may want to consider the professional fees in terms of what you are receiving rather than the labour/effort in producing it. Of course anyone could produce an estimate off the top of their head, and they may even produce the right figure - the issue is - what if it's the wrong figure, and you have invested time and money based on this. What is your recourse if you don't take professional advice?
For example a QS, assuming he is Chartered and offering professional services, will have undertaken a lengthy qualification route, will be regulated by a professional body and will (should) be backed up by some Professional Indemnity Insurance.
In producing, say, an estimate for you he may not have made much effort but you are paying for:-
1. The benefit of having an estimate you can rely on (in certain cases you may be borrowing money based on this too - and the bank will value the input of a professional 3rd party)
2. The experience of that surveyor, statistically he is more likely to make the more accurate estimate than the guy off the street.
2. The knowledge that if the professional is negligent and you suffer a loss arising from that that you could follow a complaints procedure (governed by his regulatory body) or even take him to court in the knowledge that his PI insurance should cover the awarded compensation.
When you consider this against £450 you may think differently. Of course different QS will charge different amounts and its a fairly competitive market now - you can always get a few quotes.