I think it's that kitchen above all else: very dated and looks a big room therefore expensive to change. Paint the tiles (get a professional in) or replace them with plain white with a border to break up the space from worksurface to ceiling, replace the work tops, sink - for stainless steel - and cabinet doors (or paint those if replacing the tiling) for a modest amount compared to re-doing it completely.
Also, the photos have a gloom about them, giving the place a dull look: it feels like it needs brightening and warming up with some new curtains perhaps, touches of warmer colour here and there. It's almost as though it's been de-cluttered too severely (I suspect everything is behind the doors of all the fitted furniture 
).
The presentation of the other rooms could be improved as pp have noted - for example there's too much furniture in the conservatory; it should be either a relaxing sitting room overlooking the garden or a dining room, but not both. It needs a clear purpose.
In the living room, with all the furniture pushed back to the wall the fireplace is on, it has an odd waiting room feel. Maybe a rug and a coffee table would help?
The bedroom with the gilt beaded fitted furniture would benefit from new, modern doors or freestanding furniture instead. Or even just some very simple, more modern looking beading.
Replace the yellowed bath panel with a new one.
Hang the picture in the kitchen instead of balancing it on the radiator! I don't know if it is hung, but it doesn't look like it.
Also, looking at the photos, you wouldn't know there is a downstairs WC.
Looking at sold prices, it isn't off the mark, but if the competition looks as though you could move straight in, then unless someone loved something particular about it, it's not going to shift.