We have paid some money into an account to our builders who we have used over the yrs

We pd £800 into their account as agreed before work can commence
It is now 3 weeks and we have not heard from them and the are not taking our calls
How do we claim our money back

You need to write to them, using registered snail mail and demand a response within 14 days.
If they fail to respond then file a small claim via the county courts.