As far as I am aware, the £100 fine is for returns not received by the deadline, not the payment. (unless the rules changed this year)
Payment not received will have interest added, but if its really late (not sure how late), then there may be a penalty also.
The following is taken from IR website, detailing late payment charge and surcharge.
Late payment - interest and additional charges
If you pay your tax late HMRC will charge interest from the date the tax was due until your payment is received.
If you still haven't paid your balancing payment (due on the 31 January) by 28 February, you may be charged a 5 per cent additional charge, called a surcharge, on top of the amount you still owe. This is in addition to any interest you've been asked to pay.
If you still haven't paid all of the tax due on 31 January by 31 July, you may be charged a second 5 per cent surcharge on top of the amount you still owe.
Yes, I saw that. I'm just rather surprised the accountant didn't ask for payment with the return. I'll do a BACs or CHAPs payment on Monday if necessary. 20 years ago when I started working for myself I used to understand all this tax stuff, but in recent years it's all delegated to the accountant.
yes, I know the feeling. I do my own VAT returns online, but accountant does everything else.
My accountant completes my return online and just sends his bill , and a note of how much I have to pay IR by end of Jan and end of July, so I have never given a tax payment to my accountant to send, but I guess some maybe just operate in different ways.
I left my self assessment to the last minute and finally got the figures together on Friday (29th Jan). Then I realised that I had mislaid the user code and password for on-line filing. Grrrr.
Eventually I phoned HMRC's helpline and was told that as I had mislaid both, they would have to be posted rather than sent by email. Fair enough for security, I suppose, but that means that I have no way of filing the return by the deadline.
So I checked with the helpline chap that if I managed to find the code and password in what passes for my filing system, it would still work and wouldn't have been disabled. No problem he said - it will still work fine.
Guess what. Eventually I found the code & password, tried them online and they didn't work !
Rang the helpline again, queued for another 15 minutes and was eventually told that as I had an older code number (from 2002 rather than 2005) it had been disabled and I was stuffed (my words not hers). She apologised for me being misinformed on the previous call, logged the current call and said there was a chance that my late filing fine might be set at £0 rather than £100.
Grrrrrr and more grrrrr.
PS Next year I won't leave it to the last minute - but that's what I've said every time.