Update: At their max time limit of 5 days, I finally had an email reply from E-ON. It said 'Sorry you haven't heard from us - please phone this number: 0845 301 4928'
I phoned, had a message warning of high call volumes and a suggestion that I email them...
Just as a deep sigh was leaving my mouth, it was then quickly answered by a very pleasant and efficient lady who promptly confirmed that they'd received my application on the 3rd Oct, which was the Monday following the Friday it was posted - so all good.
This is apparently the 'first' stage of the process and I should soon be receiving a letter asking for proof of purchase and proof of ID. Fair enough.
A point about 'Recorded Delivery' form the Royal Mail; a month after I'd sent the letter, I logged onto their tracking service and entered the code. Answer? "We've got your package!" Not what I wanted to hear - either it wasn't signed for, or its delivery wasn't recorded at all. Not helpful.
I'd strongly urge guaranteed next day special delivery as mentioned above - don't allow anything to muck up the application process.