I'd give them no slack on this and move to a different provider if they have not got at least as far as a sensible reply to an email enquiry to firstname.lastname@example.org
by the end of the week.
I like Ecotricity, and there are plenty of other providers who could do your FIT and gas and electricity, if EoN doesn't want to. I wouldn't bother over the phone as they might be busier than average with the cold weather, and by email you get it in writing with a legally valid date stamp.
Ecotricity emailed to my usual mailbox for 'microtricity' that they'll be paying me imminently (which is on time) for one of the summer quarters, so Ofgem have been passing through payments in the expected timescales, at least as far as the biller offices who are meant to pay you (quarterly) for your generation by the end of November. If you are with a service provider who can't do that then switch provider.
Do please check your records. Having no smart meter, I look at my FIT generation meter once every three months, click a link in an email from the provider, and type in a meter number (kWh). My rooftop solar has generated more than 22000 kWh since it was installed. Without such a number, nobody can calculate how much to pay you because they don't know how much you generated. Therefore you should log in to your online account and see the dates and meter values on record for you. If there has been a mixup and this years' readings got lost or never sent then you'll find a reading dated last year, and should type in your present reading now. If you don't have an internet computer then try asking email@example.com
what can they do? It will be two or three months before the next payments from Ofgem go through and payouts to FIT occur, at which you should get a largish payment for more than a whole year since the generation meter number which you last got paid up until.