Whenever I have done that they have offered very little (or even the same) salary as I am on.

One time I was contacted about a role in London offering between 65k and 90k (can't remember exactly, but a range along those lines). I said I would only consider moving for the 90K end. He asked what I was on. I refused to tell him then eventually got wore down and told him my relatively small Spanish salary. He refused to put me through for that saying that it was not usual to get more than 10% more than what I am on.

The company was willing to pay more for "the right candidate", but my salary at the time meant I wasn't that candidate.

Don't disclose your salary to recruiters.