Garage Mis-Diagnosis and Repair Refusal

shbd

First of all, apologies if the thread is not meant to be here.

A few months ago, my car's transmission failed, as an automatic it would not get into Drive or Any gear, basically undriveable.
Brought it to a garage for diagnosis and repair.
They diagnosed a problem with the gearbox actuator and the gearbox ECU, quoting me £800 for both. 
After taking weeks on weeks, and continuous calls and visits, they finally repaired the car, which ran for literally a few days before the problem arising again.
Took the car back, which apparently had an electrical fault with the ECU, another repair, and again failed after a few weeks. 

The repair's got a warranty, but since breaking down again, the garage is refusing the take the liability, blaming the clutch for the problem.
Surely, it would be the garage's responsibility to diagnose and repair the right part? 

Or at least, common sense would be to try with the clutch as cheaper than start with the most expensive option being reconditioning the gearbox?

The garage is now asking me to pay to change the clutch, and I'm not sure if I should cough up the extra money or do something else.

Their point is that they offered a warranty on the Gearbox and not the clutch, therefore they're not liable.
From my point, I instructed the garage to repair the car not getting into "Drive", they came back with their diagnosis as a faulty gearbox, and I trusted their expertise to give a go ahead. But the problem is still not fixed, and I'm not liable for their misdiagnosis, what I paid for was the car to be back on road.

Now, I'm neither a Mechanic or a Lawyer, so I'm asking where do I stand and what are my rights? 
Thanks in advance for any response 

AdrianC

Given that there's a clutch, I'm presuming this isn't a "proper" automatic, but a semi-auto of some flavour? These can be very, very complex indeed - and diagnosis can be far from straightforward for a generalist garage. It's also entirely possible there are/were several problems. Proving they did misdiagnose is not going to be straightforward.

But if the box was removed for reconditioning, standard practice would be to refit with a new clutch anyway - simply because the only additional cost would be for the parts.