I’m struggling to understand the layout of the property (properties). You say that the bungalow your father is living in is part of your house. So as it doesn’t have it’s own address, it won’t have it’s own council tax bill, and therefore having him living there isn’t increasing your council tax liability. You’d be paying that amount whether he was living there or not.
Your own TV licence should cover his TV also, as the annexe is part of your home. So he’s not costing you anything extra in TV licence fees.
You agreed to convert the run-down barn into a home for him, yet you have created two bungalows. Does that mean he’s actually living in a property half the size he expected? Plus you are benefiting from the rental income of the other bungalow, which has come partly from his investment/estate.
What happened to the caravan he was previously living in? Do you still have that? Do you rent that out, or did you sell it, and recoup some of your money from that?
I don’t know why you feel he should contribute towards the maintenance of the lane. He’s not directly responsible for any potholes, and has already been pointed out, you would be paying for this maintenance anyway, so he’s not costing you anything extra for that.
If you’re not viewing his £40,000 investment into your conversion as part of your inheritance, then how DO you view it? How will your siblings access their share of this money in future? If you and your husband/children will be owners of the bungalow he's currently occupying, and will still have it for future rentals, then you’re ultimately benefiting from £40,000 of his estate that your siblings are not sharing in.
You say, in relation to your siblings – “They have all enjoyed lots of holidays eating out and new cars over the last 7 years whilst we have gone without and they all have better pensions than us!” Have you done without these things and not invested in pensions because of your father? Or because your own financial circumstances didn’t allow it? You seem very bitter about this, whilst overlooking the investment your father has made into your property, and which it seems you will ultimately benefit from.
The property will have increased in value since you purchased it, and have renovated it, so you have that as your “pension” as you could sell and downsize.
Is your father fully aware of your financial circumstances? If not, then write down all your income and outgoings, and show him HOW you are unable to continue to subsidise him and that you really need him to make a reasonable contribution towards the bills. Involve your siblings in the conversation if you feel you need “back up”. Seeing something written down in black and white can often be a “reality check” and he may then be amenable to making a contribution.