I received an invoice from a landlord about a cleaning bill. I heavily dispute this (is seriously out of whack: charging 7 hours work for a 15 minute job). She's being all weird and cagey about things so I thought I'd check things out for myself. She forwarded the me the invoice which has website link (which is broken), company name and address (hasn't ever existed on companieshouse.gov.uk) and an account number and sort code. The landlady owns over 40 properties in the area and I'm starting to wonder if she owns the company and this is a little trick of hers (she's been very dodgy from the start). Is there anyway I can find out if I can determine if the account is a business account or even if I can find the name registered to it?
No, the best you can do is (probably) determine the bank, from the sort code. using an online checker such as this one from the UK payments industry trade association. Revealing the name of an account holder is something the bank would typically require a warrant for, I'd expect, or whatever is covered in the account T&Cs under "we provide all lawfully required assistance to the authorities"
Switching to what I suspect is your underlying problem - if this is a dispute that's arisen at the end of your tenancy, relating to the return of the deposit, then there are plenty of people to help you, for free. Use those rather than attempting your own detective work. Start with the UK government How to Rent guide, which includes links on to Shelter's pages about deposits. The CAB has lots of good info here too.
Note that if your landlord didn't put your deposit in a deposit protection scheme, then as a professional landlord they could be penalised four times (I think) the deposit amount by a court, so stick to your guns on this.