Thanks for all the comments.
I'm borrowing a code reader at the weekend, I'll be putting that on the car and will post the findings here.
Yes - it failed on idle "Natural Idle CO max is 0.2, measured 2.93, Result "Not OK"."
I posted pictures of the results here, but i understand that it's not always easy to see pictures on small devices.
To explain what the MoT garage i used did...
...I know them reasonably well, have been using them for years. I told them not to waste money... but they did (for free) strip down the throttle assembly and clean out the valve (although they said it looked quite clean anyway). The re-test was done straight after i'd driven it for 20 miles to work. Their guess was catalytic converter next. But given the age/mileage of the car (and the decay in the hood), i decided to pause the investigation.
I wasn't planning on using it much over winter... i need to replace one of my cars at the beginning of April, so I have until March (and £25/month of road fund tax is not insignificant.
... i wonder how much it will deteriorate if left outside (even under a plastic sheet) overwinter?
and until i get it through the emission check I still won't know whether it's worth fixing the rest.
Realistically my wife may object to it sitting unused on our driveway all winter!
Z