Can anyone tell me what the regulations are on the period Telcos/Broadband companies can go back to charge customers?

I seem to remember that about 10 years ago, they could charge you for a period going back 6 months.

I have had a bill from the Post Office for my broadband, which goes back about a year, the reason being, they had technical problems and were unable to send the bills out to some people (this is what the accompanying letter says).

The bill comes to more than £132.

Can they just do that?
Or are there rules which say otherwise?

Please fill me in.

Thank you in advance.