Remember that the crossing words have to be real words. And you can assume no letter will map to itself. That gives you very few options. There's only one option for the second letter of 13d for example. And often only 2 or 3 for others. Especially where the same letter appears several times in the perimeter (eg c).
Thanks for the pointers. Ludicrously, by the time I checked back here to find them, I did indeed crack it. I really had been unsuccessfully bashing away at this all week and then as soon as I say so, it stats to fall into place…
... think one of those that you can only confirm in chambers by looking up the tradititional spelling ... having completed ok think I have a 'blind' spot, for probably clever word play for 13A ... or 60 herein ...
To explain ... I took the first word as the answer (a verified synonym) ... then wordplay 1 letter 'about' + 1 letter 'fifty' ... leaving 3 letters for 'or 60 herein' making no sense ... Unlike me to stick to first conclusions! ... Doh