Sorry, should have been clearer. When I mentioned "on deposit" I was using that as shorthand for my total non-mortgage position with them. At the sums involved in my case I'm a million miles away from the offset product being attractive versus their tracker.
One other point to consider - old-style mortgages are the classic financial services commodity i.e. the standard of customer service is not of major concern since the lender usually finds it hard to screw up once the mortgage is up and running (keep paying your mortgage and the whole thing usually runs fairly smoothly). Customer service issues are of much more concern in the case of current accounts where poor service can really cause hassle for the customer. For me, the rate should be nearly all you consider in choosing a mortgage provider but the current account choice is more complex.
On the basis of my experience to date with NIB, while I'm still happy to move my mortgage to them, their performance with me to date (I still haven't received an offer in writing even though the loan was, I'm assured verbally, agreed weeks ago; I always have to chase them and my 'contact' is off-site more than he's present etc.) has not instilled the sort of confidence in me that would make me rush to move my current account to them.
In short, while a rate of return equal to a mortgage rate on your average monthly current account balance might make you rush into moving current account provider, I'd look before leaping. A current account run smoothly by an obliging bank official is worth its weight in gold.