Can I ask what those 40 constituencies were?
A useful quick'n'easy resource for this is here:
http://www.politicsresources.net/area/uk.htm
Scroll down to the list of general elections since 1832 and look for the ones in the format:
'1997 general election, full results by constituency, including a
statistical breakdown of the results'
On the statistical breakdown page (click through), towards the bottom is a link for "There is a list of constituencies
in order of percentage majority, together with separate lists for the three main parties in order of marginality".
The percentage majority page can be filtered on Con/Lab/LD (or Alliance or Liberal for earlier elections).
It's an excellent resource for quick reference. That's where I got the 37 Conservative constituencies with a sub-3000 majority from earlier. They are:
[FONT="]Dorset South[/FONT]
[FONT="]Bedfordshire South West[/FONT]
[FONT="]Teignbridge[/FONT]
[FONT="]Hexham[/FONT]
[FONT="]Lichfield[/FONT]
[FONT="]Bury St Edmunds[/FONT]
[FONT="]Wells[/FONT]
[FONT="]Meriden[/FONT]
[FONT="]Mid Dorset & Poole North[/FONT]
[FONT="]Boston & Skegness[/FONT]
[FONT="]Totnes[/FONT]
[FONT="]Uxbridge[/FONT]
[FONT="]Bosworth[/FONT]
[FONT="]Chipping Barnet[/FONT]
[FONT="]North Norfolk[/FONT]
[FONT="]Beverley & Holderness[/FONT]
[FONT="]Mid Norfolk[/FONT]
[FONT="]Eddisbury[/FONT]
[FONT="]Billericay[/FONT]
[FONT="]Tiverton & Honiton[/FONT]
[FONT="]Altrincham & Sale West[/FONT]
[FONT="]Bridgwater[/FONT]
[FONT="]Dorset West[/FONT]
[FONT="]Eastbourne[/FONT]
[FONT="]Suffolk West[/FONT]
[FONT="]Christchurch[/FONT]
[FONT="]Norfolk South West[/FONT]
[FONT="]Basingstoke[/FONT]
[FONT="]Shropshire North[/FONT]
[FONT="]Wycombe[/FONT]
[FONT="]Surrey South West[/FONT]
[FONT="]Orpington[/FONT]
[FONT="]Grantham & Stamford[/FONT]
[FONT="]Dorset North[/FONT]
[FONT="]Aldridge-Brownhills[/FONT]
[FONT="]Southend West[/FONT]
[FONT="]Thanet North
Note that the order is in percentage majority, so you've got to keep an eye out on the raw vote score.
Of these, 23 are Con-v-labour; 14 are Con-v-LD
[/FONT]