I've never heard of tap tds being that high. I'm in the Anglian Water region and we have some of the hardest water in UK, its usually around the 400 mark, did mine this morning and it's a little higher at 435. Even the waste water from my ro only registers around the 500 to 600 mark, 800 is just nuts, definitely ring up your supplier (presumably AW or could be Cambridge Water?) and have a chat.
Edit..and borrow a colleagues tds meter just to check yours isn't faulty.