I have written a large menu sustem in Turbo Pascal - and would like now to
convert this to Borland Turbo C

Apparently there is a convertor out there for the PC run in either DOS or
Windows

I would be most grateful if someone could help me find it, and also advise
on how reliable it is

Perhaps you can use p2c, but it is written for converting Pascal into
GNU C++. If you did not use objects in your pascal code, it should work;
you will have to change some bits in p2c.h, however.