teesside tds is around 115 (Middlesbrough). Thats what ours is In Saltburn as well.
Hartlepool gets their water from wells and that's horrendous. 485 - 550. (It had occurred to me that Hartlepool purify their water for a couple of the industrial estates to near laboratory standards. They are probably putting that waste flush water back into the water supply rather than drain it away and waste it.)
Not sure where Darlo gets its water from, but one of the wells Hartlepool draws its water from is near Darlington.
Ian's right. They only way to know is to buy a tds meter - first purchase.
When we first started our water was 255 in Saltburn, but after they had renewed all the pipes, it dropped to 90. It has been around the 115 for the last year or so now.