I should think these people are as good as anyone Jo.....................
http://www.cheltenhamglassandglazing.co.uk/
Replace before survey if you can. Don't give surveyors any excuse for a downer ;o)
Easy to replace yourself if you like. The unit should be around the £50 mark including VAT. Because it's a door, the unit would have to be of toughened glass. That makes it more expensive than "fixed" units.
If Glaziers fit it, that cost would mostly be reflected in their travelling time.