It is true the pre letting expenses are not allowd, but repairs etc between tenancies are allowed. Only thing is that June is more than 6 months ago, I'd clarify directly with revenue, by email or letter. A valid reason for the delay might be that you didn't have the money and couldn't let it in the current condition.
Windows, I consider this not to be capital. Ditto the heating. I consider capital putting on an extension, converting a unit into multiple units, a sun room etc.
Every house will have windows that need 'replacing', same with the central heating.
If you're not sure, hire an accountant, money well spent.
Sounds like quite a bit of expenditure, if you borrow the money, 75% of the interest is deductable.