OT. Sim for 93 year old with sporadic use

We've had Vodafone credit expire after three months. O2 and Virgin credit doesn't appear to expire

Asda paperwork says 1 call or text etc every 3 months required

Some of these areas are in olde Wales...

I think if you just want good voice coverage then the networks are much of a muchness - O2 and Vodafone have some advantage as their GSM networks operate at 900Mhz whereas Orange/t-Mobile operate at 1800Mhz. Orange and t-Mobile have therefore had to build more base stations to provide the same amount of coverage. Where 3G is concerned, however, all networks operate at 2100Mhz. Because most networks just added 3G capability to their existing base stations, Orange and t-mobile were at an advantage as they already had more base stations in locations better suited to the higher frequencies. 3 have built a 3G network from scratch and in the industry it is regarded as the most comprehensive, although they have now joined an agreement with t-Mobile to site share. The net effect of this is there's usually 3 coverage wherever there's t-mobile coverage and vice versa. With the merger of Orange and t-mobile into everything everywhere, their users can now roam on each others network, so O2 and Vodafone are at a serious disadvantage (unless they manage to persuade Ofcom to let them use some 900MHz spectrum for 3G)

However those that operate at 900MHz have as distinct advantage when it comes to indoor reception.

I am the OP. ASDA/Vodafone sim worked for Mum in her hospital.

Took my wife to a different hospital to see a consultant in a non-clinical area yesterday. No signal on my Voda or her Orange.T-mob unless we were close to a window. New building. Not sure of the construction but never had any problems around that hospital before.

There are no 3G networks that operate at 900Mhz in the UK (yet) though...

Likely foil-faced Kinspan panels all over it.