I've read further into it, and it seems to be quite an interesting website.
I've registered my car, and I can see the data usage on a day-to-day basis, which may assist in deciding whether to buy a second SIM instead of subscribing for hotspot use. It seems to record the data usage each time the car is started, but doesn't tell you what the data is. However for my car yesterday (9 starts) the data usage was:-
06/04/2017 110 KB
06/04/2017 58 KB
06/04/2017 169 KB
06/04/2017 481 KB
06/04/2017 75 KB
06/04/2017 93 KB
06/04/2017 124 KB
06/04/2017 400 KB
06/04/2017 138 KB
This is car-related data only, no hotspot activity.
Rates for a UK-only package from Data Inside are:-
7 Day 500 MB 5.99 £
30 Day 1 GB 9.99 £
365 Day 10 GB 99.00 £
Rates for a pan-european package are:-
7 Day 500 MB 12.99 £
30 Day 1 GB 17.99 £
365 Day 10 GB 155.00 £
Note that the data usage limit may expire before the number of days are up.
I don't think I'll be making enough use of it to warrant the annual package, but the 7 day one for when I'm on holiday should allow my wife to use her tablet while we are on the move. Providing she doesn't expect to stream any videos. Should be more than enough for a bit of email and Facebook time though.