The other way to solve problem:
"Dear Uncle Buck
I have used to landregistry to track down the Landlords name and contact details.
You have 7 days to get my Mothers name removed from the fraudulent tenancy agreement agreement and send evident that its done.
Or we will be informing the landlord(and there mortgage provider) that you have been illegally subletting.
The police will be informed that you've forged my mothers signature on a legal document as well
Yours... "
Given that your Uncle probable doesn't want to homeless or questioned about fraud... he's going to sort it out.
If he doesn't do as you ask (I think he will) tell the landlord you mum moved out 6.5 year ago and uncle promised he'd tell the landlord...