Hi all good advice, i'm a builder.
To value it look up and down the street check zoopla etc for current recent house sales and any others
for sale at moment, most streets have had a mix of houses as you describe yours and done up houses.
Check for highest value of house on street that has ever been sold and why, has it had an extension/ garage/ conservatory etc
if so get rough estimate from builder and do some maths. just bare in mind as previous comments(cost investment time stress)
Sorry for your loss and good luck.