My accountant requested the interest i recieved during the year,..I emailed the details third week in jan to his firm with his name in the subject bar,whilst on holiday in late jan/early feb he contacted work to say he hadnt recieved them luckily i had kept a copy.He said that he hadnt recieved the email.Should i pay the £100 or should his firm
As sherrard says, sending is not proof of receipt. Their server might have been down. Perhaps he didn't recognise the importance of the email because you put his name in the subject box, rather than "My interest Return" or similar, and deleted it, or it went in his Junk folder. A Read Receipt request lets you know that they received the email and opened it.
The £100 is always going to be your problem, even if the accountant was in any way liable (which all indications in the thread re that he wasn't). It's YOUR responsibility to complete a tax return and ensure it meets the deadline and no amount of hiring an accountant to do it for you removes that responsibility from you. You have to pay the £100.
If he's a responsible accountant and accepts it's his fault he should offer to reimburse you of course, though as I say, from everything supplied, I can't see how it's the accountant's fault.