Thanks for the picture.
I have the opposite problem to you.. I have a mystery frame whose serial number leads me to believe it is the 13th frame built in 1975, possibly by Stan Pike. It's not an SP serial number though.
The only Pike i've found on the web with a stated SP serial number is this one:http://bigtop-design.co.uk/blog/?p=2046
I assume these numbers are sequential.. If they are, that would mean that your frame predates that one.. by 7112 frames, if my maths is correct! That doesn't sound quite feasible, so maybe these SP numbers aren't sequential after all. Sorry I cannot be of more help!
Close up pics of lugs and fork crown and other details on your frame may help someone to date the frame more accurately, maybe not, but people love looking at that stuff, either way..