Looked out of my kitchen window around 8.30am on Monday and saw an engineer up a telegraph pole. He worked for around 3 hours and told me that my cable had become exposed at a junction box on the pole and was corroding badly, hence the problems when it got wet.
It's been raining for a couple of hours now and my line is clear as a bell. Download speeds have also improved a bit.
Thanks everyone again for your help. Fingers crossed this has fixed things. 🙂
Me again. 🙂
The noise on the phone line appears to have gone but I've had 3 disconnects in quick succession tonight - it's windy.
Could branches catching on the line be causing this? The cable goes through quite a few trees close to the house.