How do I get a valuation?

[superlambanana]

We need to remortgage as our current extortionate fixed rate is coming to an end. We will be merging two mortgages - one that was on fixed rate, and one that was a top-up when we upsized.

Since we bought this house we have had a big extension, that is due to be finished in a month's time (then we start decorating!). The rest of the house, bar two rooms which are done, also need decorating as we had it all rewired as well.

We know what we want it to be valued at when it's done, based on how much we've spent on it and what other sized houses are going for. But I've no idea what it's worth now, and I don't know if mortgage companies would value it based on what it will be like (in a couple of months' time) or on what it is the day that they see it.

Do I ask estate agents to come round? If so should I be honest with them or pretend that we are thinking of selling? Do they get requests for valuations? I know the mortgage company will value it but we need a starting point to know what amount to apply for (obviously the higher it's valued the better deal we will get). We are on a deadline as we have to do it before I go on maternity leave at the end of May!

Any advice appreciated...