Thanks for the kind words guys - but it's not all down to me! If one of the other owners I know didn't have the knowledge to do the programming to make the arduino recognise the transmission (I've tried numerous times and failed! but then my coding on Arduino is somewhat limited!) then I'd still be stuck with the idea (which I now know works, which is great!) and no way of implementing it.
Final price/availability will depend on how cheaply we can make them once a working, low power prototype has been tested - but there is somewhat a light at the end of the tunnel now on an alternative to 2 fobs (which is a great, cheap workaround to the challenge LR has presented us all with!) or going to the dealer to pay through the nose for a little box to fix a problem they blatently knew about.
That aside... another project I've been working on... I appear to be able to repair the later WABCO 'D' ECU's, SRD100290/500/501 if they have a 'Sensor short to another sensor' fault... most commonly "LHR sensor short to another sensor" I've had success with 3 ECU's (including my own P38's which threw a wobbly a few months ago) and 2 others which have already been snapped up by other owners - on exchange for the faulty one back.. I've got 2 more on my bench I've repaired today, and will road test them next week - but hopefully that's another part which is now repairable - and have an alternative to a second hand unit from a breaker vehicle, or a brand new one at £850 odd!
I'll probably make a new thread on that at some point once I'm confident the repairs on the ones so far isn't just a coincidence!