It's 2 or 3 years since I last got it, as I tend to holiday in spring and autumn, and I only usually get up early on Sundays if I'm on holiday or going on holiday, so the times when this service would be useful to me tend to be outside the period when it runs, which is why I frequently complain about it only running in summer

But last time I did get it, it was pretty busy (more than 50% of the seats taken on a 4-carriage train) and that was still during the post-Covid recovery period. So not as busy as the first Sunday train from Ely to Kings Cross - which comfortably fills almost all the seats on an 8-car whenever I've got that one - but a decent loading for that time on a Sunday morning.
On a related note, it'll be interesting to see if this first service to Kings Cross remains as busy as now if it is indeed timed 40 minutes earlier from December, as it was in the draft timetable. I'm guessing people may split between that one and the one an hour later, but a London arrival at 0933 is rather more appealing than the 1011 that is the earliest currently possible.
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The later start seems a bit unnecessary - over half an hour later than currently. To be fair, I can't say how busy the current 0908 off Ely is, as I don't think I've ever got it, so maybe this isn't something that matters all that much.
I had a feeling there was always an irritating gap in the middle of the day on the east side of this service, but the earliest NRT PDF I have to hand is from December 2014 and there all the services in the middle of the day ran through to Norwich. The May 2007 printed NRT has the 1352 ex-Liverpool only running to Nottingham, so maybe that's what I'm remembering (time flies!). I don't have anything in between to hand to see when that was extended to Norwich.
But yes, this is a gap which needs filling, and if not at this change, you've got to wonder when.