every time we tried to clear the fault codes ( DTC's ) they would return immediately, this included SDD ( Sympton Driven Diags) scanner, the latest Snap On, and many others since April

we tried to troubleshoot this since April - since it can be a MOT failure
so a new battery was installed back in April when we had the MOT done, on a CTEK trickle charge
the voltage was proven to be good to the SRS DCU - so no problem there
all the sensors located and cleaned and checked all wiring - there are 8 crash sensors I think
loads of other rather pointless and stupid things to go through which really has no bearing on the problem - hey ho

finally decided to swap out the SRS DCU today with one that was exactly the same Bosch part code from a 2004 Range Rover

Click image to enlarge

We've had a few like that - won't clear DTC's, different marque but similar age - put down to 'internal non-recoverable defect, replacement required'. I'm sure that some of the vehicle modules have a counter on the number of times you can clear the flash memory before it locks, I've seen it on all kinds of modules, usually the one with a history of faults stored over the years...

thank you for your guidance, I will be honest with you, I have been putting the job off for 3 or 4 months, but wanted it done well before winter, and we will see if it holds up until the April 2023 MOT

I'm sure it'll be fine