I have horizontal shadows that spread across the screen. It's gets worse with darker text or blocks of colour. It scrolls up and down alongside the text etc.

Advice on how to correct this would be appreciated.

Thanks

Have you got anything that could cause interference plugged in near it, cordless phone etc?
How old is the monitor and do you have a spare you can try to see if the problem is still there?

Monitor is about 4 - 5 years old and I have a cordless phone nearby which I have unplugged - that made no difference.

I'd see if you can try another monitor first, borrow one.
If its 4-5 years old, chances are its on the way out. The rule of thumb is not to have any transformers near a monitor as they will cause interference and damage over the long term.