O2 PAYG SIM - GPRS Included? Settings?

I have a PAYG O2 SIM. Top up £10 a month and get 500 texts included.
I thought that this included 1mb of GPRS browsing but, having had the
settings sent to me via the O2 site, GPRS access works but I am charged
for all access.

Is GPRS not included in these PAYG SIMs? If it is, could someone point
me to the settings to stop me being charged?

Many thanks