late fees was I allowed??

I have been doing the plumbing for an estate agents, on each invoice I always put please pay strictly within 30 days, no mention of late fees, they always paid late. I warned them via e-mail that charges will start to be made unless I was paid ontime, still no payment. I started then charging them £40 per invoice (4 outstanding) and they say they are not paying it because they never saw my terms and conditions? was I right to add charges and can they do this?