TL:DR elec company seem to have been carrying all debit balances into next bills but not carrying all credit balances over (esp larger amount at annual review times)
Got notice of an annual review & uplift in my d/d but am in currently in credit so was confused as to why.
Rang to find out and got fobbed off a bit so I asked for a further review and hope to be called back tomorrow.
So meantime decided to check all bills for last year - and discovered that last annual review my credit from the previous year had not been brought forward 
I’ve downloaded all bills for last 3years and found the same has happened before 

So now I’m having to manually check each bill 
It seems (to me) that having been on a ‘pay each bill in full every month which leaves zero balance before the next bill’ set up, when I’ve moved to budgeting plan/direct debit they have not carried forward any payment over usage into the next bill
i.e. £30 d/d but £26 usage should mean £4 credit carries over to next bill (so £30 d/d+£4cr = balance £34 towards next bill)
They’ve carried over debits (when I’ve paid £30 d/d but used £36 in elec the £6 not covered runs on to next bill so next £30-£6=£24 credit to come off next bill)
Essentially if I use on the dot £30 all well and good, if I use more than £30 the debit excess carries on to next bill (as it should) BUT when I’ve used less elec than £30 the company have just pocketed the credit excess
instead of carrying it over to come off the next bill.
Anyways I’m hoping to get it sorted (& all money/credit acknowledged or refunded) but with rising costs I thought I’d post to remind anyone who hasn’t checked recently to have a look over your d/d payments Vs actual usage on your bills and check that any credit is being rolled on to next bill.
I used to be right on top of checking (after a previous company balls up!) but I had a terrible couple years when I was incapable of checking - now wishing I had asked my support worker to help and I might have caught this issue sooner - and worried less than I have about money.
At least I’m lucky in that I am owed money rather than owing them! but I suspect they might have put a bit more effort into identifying the problem if it had been the other way about.
Anyone else had good (or bad!) shock at reviewing their d/d bills?