Failed credit check but scores all excellent- don't understand

[code]

DH and I applied to our current bank to increase our mortgage by 50k to fund a loft conversion. This will take mortgage up to 200k on property worth 700k. joint wages in excess of £120k/year.
We were told our application failed due to credit check and that we should go onto experian, call credit and equifax to check as those are the ones they use. Well we've just done that and credit is excellent on all 3 sites for both of us. We've lived in house for 10 years, registered to vote, been at respective employers for >10 years, both have credit cards kept up to date etc.
what is going on here? How do we address it?

I have absolutely no idea but am curious to see the other more knowledgeable answers. I have an excellent credit score but can't get a 0% credit card. And no, I don't desperately try for it, but it would be handy.

We have had exactly the same thing happen.
No idea why.

It seems crazy. I think we'll go into the branch to check. Did you do that?

Do you have much other debt? Do you have a lot of credit available eg high credit limits on credit cards? Have you made other credit applications recently?

The branch won't have any way to find out the details, however they should be able to refer to the underwriters to have the decision reviewed.

No, a few thousand on one card but other than that- nothing.

Experian couldn't even say I have lived at my address for in excess of 25 years, and that's with being on the electoral roll and having a credit card and the bills in my name. I don't get it either.

Says no credit searches on mine, will get DH to check his.