Yes, we've tried the suggestion above, as well as trying to physically budge it and attaching a hosepipe (our water pressure isn't good).
We won't be having the whole house flushed - it's prohitively expensive for an isolated issue.
We will see if there is anyone who can treat the isolated issue e.g. with a pump.
I note the people on bumpers farm do powerflushing so I'll ask them if they can deal with an isloated issue or whether they will only do the whole house.

Merry Christmas everyone