Where do I start?

so 40 pounds for the frame...(first fixed gear... no need for anything fancy)

something like this??

http://cgi.ebay.co.uk/BANKRUPT-700C-WHL-ROAD-RACING-BIKE-ALLOY-YELLOW-FRAME_W0QQitemZ170187999346QQihZ007QQcategoryZ22679QQrdZ1QQssPageNameZWD1VQQcmdZViewItemQQ_trksidZp1638Q2em118Q2el1247

or buy a cheap road bike for under 100 and strip it ??

You need an older bike with horizontal rear drop-outs so you can get the chain tensioned - something like this:

http://cgi.ebay.co.uk/GAZELLE-653-L-WEIGHT-23-ROAD-FRAME-AND-FORKS-HEADSET_W0QQitemZ300214829195QQihZ020QQcategoryZ22679QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Which will probably go for a bit more than £40 - that was a few years back. Inflation and all that.

You could buy an old bike and strip it, but make sure it has the horizontal rear drop-outs.
[URL="http://www.sheldonbrown.com/images/spokeflop.gif"][/URL]

(Chris - that bike went on Ebay)