Water is trapped behind the dashboard and it sloshes about from side to side when I corner. How can I solve this problem? Problem is worse after it's rained hard.

Can I assume there are drain holes that are blocked? If so, where are they, how do I get access to them and drain them? Would using an old curtain flex be of any use? 

Garage says it's a lot of work to get behind the dash, is this correct?

Many thanks, Birmingham1

@Birmingham1 I'm guessing slightly here, but I think the water is more likely to be in the scuttle plenum (beneath the trim the wiper spindles poke through), but yes the issue is likely to be blocked drains.

Most blocked drain holes can be cleared with wooden kebab sticks or plastic knitting needles (don’t use metal as might scratch and lead to subsequent rusting). Blocked pipes will need something flexible 

 

Try accessing from top (by the wipers), less likely to be inside dashboard 

 

 

On an old Clio, I have used the tool electricians use to pull wires with. Kind of long, thin nylon cable you can gently feed through. Might be useful here, too. Screwfix or toolstation will sell them, quite cheap and easy to locate. Cheap basic one, not elaborate pricey stuff.