Thanks 9eor9 & Petkow for your responses
A friend suggested it might be because we are on the low tariff.
Unfortunately we are stuck with YOIGO for the time being, or change to ... Movistar..... well,

I've some stories about them... We can get a weak Vodafone sig up the land..but we wont go there either!
It would be fab to be able to connect everyday, but not essential. 2 days a week I can get access in town.
I had big plans for a little structure up on the land (suitably camouflaged) where the good signal is. Inside a Zoom Travel Router.. with dongle attached..., car battery to power it, charged from a small solar panel, to broadcast wirelessly to the house. However, until we get a better, stable signal, it's not worth it.
I shall call YOIGO on 622 and see what they have to say for themselves re their shoddy service
Thanks again.