GDF,
Have a look at CAP 639 on the CCA website. It's the examination document used for CPL and ATPL performance exams and has some generic performance charts and graphs for SEP, MEP and Medium Range Twin Turbine.
I believe the MEP ones are based on Seneca's. Good enough for test/instruction, no use for flight obviously.
Link to document is here:
http://www.caa.co.uk/docs/33/CAP698.pdf
Hope this helps,
Cheers,
Vfr