UPDATE: Sincere apologies for not updating the thread. The car issues have basically taken over my life whilst I tried to get them resolved in time for Spain; ironically, that may now not happen due to the new government advice 🤦♂️
As stated above, the issue, in the end, turned out to be the replacement solenoid that I bought. I admittedly know very little about the workings of a car sadly, and therefore rely upon garages to tell me what this issue is. However two Lotus garages and an Auto electrician could not source the fault; this was purely because they tested everything but the new part I ordered. I only realised this after the fact, but in my day to day life of IT repairs, I would have tested everything personally.
I was at a loss and called my regular mechanic, but he told me he would be busy until around tuesday/wednesday of the next week. With the impending trip looming, I couldn't really wait that long.
In the end, I called around several local Auto Electricians to find someone else, and one got back to me. He recommended a guy around 1 mile from my house and when I arrived, he tested all three solenoids that I had in my possession and told me "they're all f********" lol. In the back of his garage, he had a solenoid lying around from a Vauxhall Vivaro van. We fitted it, and the limp mode disappeared as soon as I'd cleared the codes. The car let me rev all the way to the red line and cruise control stayed on etc.
It was so obvious in the end, I just had no idea the other mechanics hadn't tested the new part!