Thanks for your advice, I think i'll just leave my offer on the table then and see what happens. When they told us of the first offer, we just let them know we were interested and left it at that until we could find the offer in the paper, they kept ringing, so hopefully this will happen again.
We have thought it all through and my annual income alone is almost the value of the house, so we have left ourselves plenty of room for any changes in the future. So all should be well!