Hopefully the OP has had some succes in finding the source of the problems. It's always good to get feedback on advice given willingly.
Had a call late morning to say they started work on it today after receiving the master and slave cylinder which they found were leaking, so it looks like it was lack of fluid causing the problem. I usually check all the fluids at the very least monthly but normally more often than that. With the very wet weather over the past few months, and the fact I've hardly driven it this year, I've neglected it or I might have picked it up sooner.
Had another call later on to say two other problems had come to light. One of them is that it appears the gearbox oil has never been changed. It is very black and Johnsons checked all of the service records for this car and there is nothing in the record about the gearbox oil being changed.
The other problem is, and I can't remember exactly what she said but words the the effect that a bar/link or something on the clutch pedal was snapped and that is a little over £100 for the part. After I said they need to do it so go ahead she told me they won't charge me labour for that, just the part. This was interesting because just as the gears were getting difficult to engage there was a sharp CRACK when I depressed the clutch pedal. It didn't happen again but I guess once it's broken it's not able to break again!
Ref the gearbox oil, I was really p1ssed off about that because Bristol Street Motors, in Cheltenham, persuaded me to take out a 3 year contract for servicing saying that it works out about the same cost for the first two but the 3rd one would be a big service and the contract would save me a couple of hundred quid. The contract would take me to the 5 year service which would require a gearbox oil change. When it came to it they said they did an annual service but nothing major was required. There ensued a long argument over the telephone and a letter to Bristol Street Motors HQ but nothing happened, and that's when I started to go to my mates garage off Eastern Avenue.
So it looks like in all the 13 years the gearbox oil has not been changed, got to give it to Johnsons for picking that up. It's going to cost me but at least I'm sure they are doing a good job.
As I said to Chelsey, the young lady on servicing, I'm going to have to keep the MX-5 for a few more years to get my money back for all of the work done in 2020!