Hmmm.............

From their terms and conditions - which I hope are below -  there is
probably a reasonable expectation that you would expect to be able to go
overdrawn by up to £10:

6.6 You should ensure that your Account is in credit (i.e. positive
balance). However, upon request or registration for our "Auto top-up
facility" we may allow an overdraft of up to £10 on your Account.

However:
6.7 You Account should not be overdrawn by more than £10 for more than 3
days, if this happens we reserve the right to suspend your Services until
your account is back in a positive balance.

6.8 If we suspend the Services in accordance with condition 6.7 above for
more than 7 days we will send you a reminder by SMS and email and charge you
a reasonable administration fee for this reminder.

Seems to give them carte blanche to allow you to go overdrawn and start
charging surcharges as soon as they see fit.

But, nowhere, does it reference any appropriate consumer credit legislation,
and if you son is under 18, I believe that he is incapable of  legally
entering into a credit agreement - which this seems to be - so if you want a
fight over £30, you may have good grounds to do so sucessfully.  If,
however, you acted as a guarantor, then there is implicit acceptance of a
credit agreement and so you're probably stuffed - so to speak.

I'm not a lawyer, nor should this be taken as any kind of legal advice, but
unless you query this today, you will end up having to pay and probably a
lot more in surcharges.

Good luck................

http://www.easymobile.com/selfcare/emcontent/company/terms2.html

Thanks for the advice. Unfortunately the bailiffs are after the money in the
next 48 hours so I'd better pay up