How rude you are
Get real. You said a MkI Escort, not a hacked about heap. Yes, I got more than that from my 1970 Mk I Escort, and even after fitting a modified cross-flow 1500 GT motor in it, it was over thirty if I regulated my right foot. I didn't hang about, either, at times. My 1200 Cortina gave around 25 mpg. That was certainly better than a mate's cortina - he fitted a big V8 chevvy in his. You apparently needed to know how to set them up properly.
From the net: Average salaries were around £2,000 a year in 1971, with the average house selling for £5,632: That is about double your twenty quid a week. Someone was pulling your....
So not rude at all just quoting some better figiures to replace your rubbish ones.