Just as a suggestion. Mobi-data provide a data sim with 1GG /month for less than £5 a month with 2GB for the first 3 months. It is also not a rolling contract so can be cancelled any time.
click here have also found them very reasonable to deal with. I overused my 1st month 1GB allowance by 1.4GB as the advertised email alerts at 50%, 80% and 90% did not arrive in time due to the Easter public holiday long weekend. This could have resulted in an overcharge of over £100. I appealed to them on the basis that the emails had not arrived and they finally agreed to just cancel my second month data allowance and drop all the extra charges. I consider this to be excellent customer service.