the MOST expensive charge for interpreters come from sign language (mainly because it can't be done over the phone, like many other interpreting situations)
yet people (of course) wouldn't say we shouldn't provide that, and the budget figures get skewed because everyone look at an interpretation budget and makes the assumption that it's cause of "lazy foreigners coming over here, wanting our services but not bothering to learn our language"
To be fair though bednobs I'd hazard a guess that the services of a signer would be negligible in relation to the costs for those requiring language interpretation.
I've heard of one instance during my 21 years in the NHS.
That's not to say they're not required, just far less often.