At the end of the day you have a 3 bed, 3 reception (albeit a bit open plan) house with a shower-room, bathroom and another loo.
A different suggestion but, since you love the house, could you stay put if you switch the upper floor around a bit?
Get rid of the mezzanine double bed and make that part a playroom (stairgated, obvs). Put the older child in a bed below.
Put the twins in the long, thin bedroom.
Parents/inlaws can stay in a Premier Inn/Travelodge (or swap a sofa, for a sofabed in the lounge).
Nobody else has commented, but I also found it odd to have a roof gutter at the front, just spilling water onto the pavement.
If you don't want to drop the price by £20-£50k to find a buyer or pay for a better estate agent (I have no idea about your local prices), would you spend £1k getting an architect to suggest how to remodel some of the spaces to make them work for 5 of you?