How can it be deemed legally served with no proof it was posted other than them saying it was posted? Anybody could 'legally serve' whatever they like just by claiming they sent it first class, surely at the very least proof of first class postage is needed?Mail sent by regular first class post is deemed served for legal purposes, no proof required. That said, whether you have to pay in your particular case is another matter. Check the terms of your contract and seek legal advice, because IANAL and probably YANAL and everyone else here INAL and if they are they won't tell you for free anyway.
OP - had the same with my estate management company, no invoice received and the alleged second one had a late fee attached, was told it would stay on file but they would not chase payment for it and argued that the schedule of charges was in my info pack, they were exceedingly stroppy with me about it from the very start, it turned out my solicitor had failed to provide me the full pack, useless.