So, it has been 10 days now since the switch and everything has gone to plan. G.Inp and what I believe to be full speed was achieved about 40 hours after the switchover (just after midnight on the Wednesday after a 9.30am switch on the Monday).
Overall, the trickiest thing here was getting quality information from Zen and being sure that their systems would cope with the change, but once that was done everything went fine. Pings are a little higher on FTTC (12-14ms) rather than G.Fast (10-12) and, of course throughput speeds are much lower (29 meg a sec on G.Fast to 9.24 on FTTC), but in terms of the majority of my day to day usage there is no significant difference and it is only when downloading a game or a large update from Steam that I notice anything. I've gone back to my trusty HG612+AC68U and have saved about £300 a year and I might add to that by selling the MT992 on the bay if they are still going for a few quid.
Hopefully this thread will help someone else if they ever decide to do something similar in the future or go for a G.Fast product and wonder if they will ever be able to make it back to FTTC after their minimum contract expires.
Thank you, as well, to all of those that have offered their opinion and advice on this thread, it has been very much appreciated.