Posted: 19/05/2017 at 22:03
Sweet Millions is an F1 variety, which means the seeds come from the crossing of two (or more) very specific varieties. Because you have collected and saved the seed from last year's crop, what you now have is an F2 variety, which is likely to be similar to one of the original parents and not necessarily much like 'Sweet Millions'.
I doubt your compost mix is the problem. It has been a bit of a cold start to the year though and a bit too cold overnight in an unheated greenhouse for tomatoes so they are probably just sulking and should start to romp away now the night temperatures aren't going down so much.
I would pot them on and bury them up to the first leaves which will allow them to produce more feeding roots from the stem.