The seller of the Freedom Talk sim card kindly refunded my payment pro-rata. The next stunning deal that I've found is 6 months on T Mobile with unlimited browsing and a ½Gb monthly download limit for £17. I've got the sim card but I've not mananged to get it working yet.
... Further to the above, it's working now. I just needed to set the "APN" (please don't ask me what an "APN" is) and make it the default setting for my dongle. The seller seems confident that it'll last for 6 months. Stunning! I'm unsure of what's a browse and what's a download but I'm up and running with this deal.