HTC One... according to
this it is openable but at great risk of damage. The reviewer in the link gives the HTC One only 1 mark out of 10 for servicing access.
According to a CNET review, HTC did not consider battery replacement as necessary.
700 Euros... As for the replacement cost the .es variant of that well known South American river has them on sale at prices starting from 459 Euros...
Not much help I'm afraid but you could save about 250 euros if you really have to replace the phone with like for like.
Being the eternal cynic, I would consider this to be a lesson well learned and I'd be looking at competitive products...!