My immediate impression was that there's so little light, the downstairs in particular seems really dark and thats with the lights on! If I were a prospective buyer I wouldn't have looked past the first few images.
Sitting room - repaint white or a similarly fresh and neutral colour. There're too many brown tones for selling. Consider replacing the light shade for something lighter and more open.
Bathroom - can you repaint the bath side? I'd repaint it white. Hide away all bottles etc. A fresh house plant on the window sill would look good, one with fresh acid green leaves ideally. A spider plant could work. Paint the walls white, the current colour is dating it I'm afraid. Consider taking down the wall cabinet if you can cope without it. Consider removing blind over the bath. It blocks the light.
Kitchen- beautiful room, really beautifully done. The units do make that side of very dark though. I'd do what you can to lighten it up. White flowers, mirrors..something. a big mirror on the wall opposite kitchen which is currently blank might bounce light. Some strategic house plants - think fresh green, delicate leaves. Couple of tall ones.
Hallway - scrap the rug, replace the mirror. They first looks messy when the kitchen and hallway walls are such an elegant colour. It's also too dark. The mirror frame is miles too heavy and really weighs the space down, replace it with one that's frameless or a narrow modern frame. Consider replacing the unit below it with a thin console table with a house plant.
Garden - take down washing line, tidy away mess and tools eg dust pan. Is that a gazebo frame? Unless it's in active use, take it down. Look at borrowing a power washer and using it on your patio, think that would make a big difference and make the patio look as cared for as your lawn, which looks great and well tended.
Children's bedrooms - need to be tidier. Blitz those shelves. Put into storage anything not currently in use, dress them for photos. The wall-hanging by the window should come down, it darkens the room and hides it's proportions, it's distracting so viewers can't appreciate the size of the room. Neaten up those tv wires.
All beds - sorry but they're a mess. They should look crisp with ironed bedding.
Ceilings not low!! What an odd comment. We are are a super tall family
This is such funny response! 
I see where the pp is coming from, are any of the ceilings painted off white? They need to be white or they will appear low and also darken the space.
Finally, change estate agents. The ad is poorly written so if you're estate agent is being sloppy in that, they're not providing a good service with anything else. They should also be able to advise you on how to make it a more appealing prospect.
Your home looks like a lovely and spacious family home but you need to make it look a bit more show-house. Give the rest of the house some of the attention to detail you've given the kitchen/dining room..at the moment the property looks like its halfway through being updated. Nothing wrong with this of course, but I suspect some aspects are dating the other rooms.