Many thanks for all the help and suggestions.

I should have mentioned that an old friend sent me the tracks as MP3s; he'd already copied them from the tape pretty well.  (We made the tracks together in 1978 when our band was signed to a label here in the UK to make an EP).

I've made a lot of progress with sweetening things because the waveforms had some headroom, all except for one track which seems to have been 'mastered' differently from the rest and a lot of its waveform is maxed out, making it pretty well impervious to EQ-ing.