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Well, I've had a look and the choices are:
A pretty knackered old TA 34t ( it must have a couple of rides left in it ), which you could have for nowt, and it might fill the gap enough for the tensioner to work. Or I have a decent 36t Gebhardt SS ring and cheapo 18t cog that you can have for not very much.
Feel free to bring the bike over before the weekend and try them out for size.
Cogs, normal Shimano spline. She's currently on 52gi/32:16, but there's a lot of skipping under power because there's too much chain slack to run the tensioner in the push-up mode. I think I have a 17t from a cassette somewhere, but an actual SS sprocket would help. The chainring is ramped as well, so that would be good to replace but I don't think that's the root of the issue.