1. The cable to the garage was in situ before we moved to the property
So, its wrong.
2. The run is about 30mtrs.
I hope you don't want to run too much off this circuit. Because of volt drop
3. The cable is underground and appears to be basic 2.5mm
Twing and earth PVC cable is NOT suitable for anything than fixed wiring inside buildings
4. I realise that it doesn't meet standards but I would prefer not to have to excavate the patio to re-run the alternative higher grade and armoured cable
Well, tough doo doo. Its wrong, its dangerous and you are now taking responsibility for it.
5. It is terminated in the garage on a bog standard double socket.
Handy
6. I don't have a problem with fitting a rcd c/u in the garage but I thought I should protect the cable just in case anyone in the future dug up the ground and hit it.
Over current protection such as you have is all that the cable will require. I would not want it connected to teh RCD side of my house, as an external circuit is very likely to trip your critical circuits in the house (eg freezer, wine cooler, etc)
7. I am not an electrician, however I do have C&G finals in Telecomms which includes electrical engineering and have fitted a few ring mains in my time prior to part p.
Yes, well, me too. and I did 10 years with BT Telecom then another 10 years in data comms before deciding to take up being an electrician. I did 3 years at night school before considering myself competent to risk other people's lives with fatal voltages rather than 50v DC.
And PS. The term is ring final circuit. Ring mains are to do with water distribution systems.
If you are doing it yourself:
You will need to rip out that T&E as its not suitable and install an appropriate sized length of SWA cable. This will need terminating and testing in accordance with BS7671. And you'll need to notify the local authority before starting work.
Or find a local electrician who can do teh tricky bit. I expect he would welcome you digging a half meter trench up your garden.
Sorry chap. That's the deal.