voda one is spot on here. to the extent of it showing a no signal area in my garden and me getting no signal whatsoever in the garden yet still getting a 2g one in the house like they say. god knows how they worked that one out
It's probably a fluke - cell coverage changes per season, weather affects it, cell coverage shrinks and grows.
Your options are limited OP - You could get a pico-cell but voda will probably charge you for it as you're a low value customer. You could sell your current phone to offset some of the £150 or so you have to go and get another contract.