Thank for all the replies on this. I haven't ordered anything yet, but I am now leaning toward sticking with the NTE5 with Mk3 faceplate.
I was waiting for my ISP to reply re the official price of a call out with fitting of NTE5c, but my query has been ignored. An interesting customer service choice by the ISP since we're just out of contract...
I have swapped mine... ...2.5Mb increase on download but...
While trying to skim through comments about the new design I have seen a handful of remarks that marginal speed increases occur in some cases, but perhaps more significantly there are comments about fewer errors.
Fewer errors would be a boon, we're rural enough that interent connections in our area have always been low quality.
You couldn't pay me to swap from an NTE5A to an NTE5C... ...I recommend using an NTE5A in combination with an OpenReach MK3 SSFP.
it's just a shame they end up sticking out the wall so much...
This is an annoyance, our master socket is in a bedroom and the overall room layout means we need/want a chest of draws in the corner so that it opens onto the socket. A real PITA and I already have a chock of wood wedged into the corner to prevent anyone pushing the chest of drawers up so far it would smash the socket.
Since I am leaning toward getting the Mk3 I guess I will be looking for something else to fill the space and push the drawers further out.
OP's crux query...
I think it fair to say that Openreach don't have very accurate records as to which type of faceplate is fitted where...
If you're reasonably competent at wiring...
I'm pretty sure I could wire myself without issue, there's just that teeny tiny nagging doubt from comments I've read before that if something goes wrong it's possible to trip or blow stuff in the street cabinet or at the exchange. Maybe it's just scaremongering to put people off, but I can't say I know for sure.
Disappointingly I can't find anyone in my local trade ads so far and the idea of £120+ for Openreach to do such a trivial thing really grates, so all considerations are pointing toward the Mk3, despite the potentially improved connection stability that may be offered by the Mk4.
Thank again everyone for the advice.