Better to confirm it before next year because while from the information we have it doesn't look like there's a taxable gain, if there is you're already late so you want it to be as early as possible. You could probably wait til 1st Feb but don't leave it til next tax season.
E.g. you said you had the house for 56 months, but it looks like it should be 75 months? So you need to get all your facts straight.
I think I agree there's unlikely to be a chargeable gain here.
Conceptually I would calculate the tax like this:
Sales price (417k)
Less costs to sell (unknown)
Less upgrade costs (80k) *
Less purchase price (255k)
Less costs to purchase (I think including stamp duty) (unknown)
Gives gain (Max 82k between the two of you)
In reality with will be less than that by a few thousand.
Then deal with the proportion of the time it was your private residence just ordinarily so in this instance that would be 75 - 17 I think but you'd need to correct that for actuals. So 58? Or 56 months depending when in the various months you moved (assume this might be the 56 months you ref in the opening post).
So that portion is definitely eligible for relief :
56/75 x82 = 61k ish eligible for relief. In reality it might be a couple of months higher and the 82k will be a few thousand lower.
So then there 22k that you're not sure if you can get relief on.
There is then more you can do, but this is the point at which my confidence in the calculations runs out. Even at this point that's only £11k chargeable gain each, on which you each get a cgt allowance of 6k so only 5k each taxable, (I think) at either 18% or 28% depending on your tax band, so £3k total.
Obviously you'd have the issues arising from late payment if you determined you did have to pay.
BUT it's then actually still probably okay because then you start applying the various rules being referenced up thread - topping the final live there up to 9 months, disapplying gaps under 3 years total etc etc. (I get very hazy on those bits @Taxdoesnthavetobetaxing has been much more helpful.)
In all likelihood you get the chargeable gain down to nil or at the very least to below your collective allowances for CGT.
- Editing to note you need to check what's an allowable upgrade cost as well, I've assumed this all is