Thanks for the "heads up": I've just found the info on their website here: Vodafone Data-Only SIMs
I've just got a spare Vodafone SIM so I'll pop in in the router tomorrow, add a bit of credit and try it out!
£25 per month for 50Gb data per month on a 30-day rolling contract is not bad at all - will be interesting if I can get a decent 4G signal in the house.
I do know that EE do special offers from time-to-time, when I last spoke to their TeleSales they said they usually do a 64Gb per month for around £30 (or a little under - discounted further if you're already an EE customer) every so often, at least twice a year - either on a 12-month or rolling 30-day contract - which is even better value than the Vodafone 50Gb offer. (Plus I know I can get a reasonable speed on even 1 bar of EE 4G here). I'll keep looking out for the offers.
But - in the meantime, 50Gb for £25 on a 30-day contract is very tempting (and, as it's just a 30-day rolling contract, can't really go wrong) - will test the signal tomorrow - thanks again!