Exactly Yorkiegirl - so with ours, we've effectively got savings of £18K but it's in with the mortgage so we only pay what we owe. BUT, if we need to access any of that money, it's just there.
So if you had a mortgage of 150K and you had savings of 25K you're account would be -125K but you'd have the facility to go up to -150K without seeking any permission, you can just take it out/spend it (if you wanted!).
Prufrock is right though and because I'm self employed, I have chunks of money going in every so often. DH also gets paid 13 times a year so every 4 weeks and so that helps too. Our aim is to chip our mortgage down and down and we are finding this is the best way to do it.
BUT, like Prufock says, you can just have a normal flexible mortgage where they allow you pay off a certain % of that each year so you could keep your savings separate until you wanted to use it to pay that chunk off.
SOrry, waffling!