Sorry about that…. Popped out for longer than I anticipated, then forgot what I was doing…
Buy an unlocked dongle from amazon/ebay/similar, then you can use any old sim cards you like.
The cheapest way I have found to do it in this country is to by the 3Mobile sim cards preloaded with 3gb/3months of access. These just stop working when either you have used all the data or you reach three months. You can then just buy top up data/pay as you go or similar… Howevere, the cheapest thing to do is just to chuck it away and put a new one in with another 3gb/3 months worth of data preloaded. They currently cost £21 from 3 or £11-odd from Amazon. Buy them from amazon and it works out at £3-4 (per month/per Gb). I just buy 2-3 at a time, keep one in my iPad (which doesn’t need a dongle) and keep a spare tucked behind the cards in my wallet. If the one in my iPad runs out I just chuck it away and put the next one in.
Slightly more faff but much cheaper.