Shared ownership: Flat won't sell - buy 100% - remortgage or let it out?

It's a bit complicated so I'm wondering if I need to see a mortgage advisor but thought I'd ask here first

I moved out of my flat two years ago to be closer to family after having a baby. We are renting elsewhere and the cost of paying mortgage on one property and rent on another (plus the associated council tax etc) is crippling us. I have a 75% share in the flat with the other 25% owned by a housing association.

I've tried selling it and there has been little interest. I've tried two estate agents and have dropped the price as far as I can go but the market is just stagnant (Preston). It's now on at £20k less than it was when I bought it 10 years ago (86k compared to 105k).

I'm not looking to make a profit but I'd hate to lose the £30k equity in it as it could be the deposit on a home for us.

The housing association has agreed to sell the final share to me so that I can at least be free of the extortionate rent on the flat (a 100% mortgage will be considerably less than the 75% mortgage plus rent).

I'm thinking it might be better to rent it out so I can at least cancel out some of the costs involved.

So...

1. Do I just buy out the final share using my current mortgage provider (Nationwide - good rates but doesn't do buy to let) in the hope the market will pick up and it will sell soon.

2. Go to another mortgage provider with the hope that they will let me rent the property out if it doesn't sell (I'm thinking of setting myself a three-month deadline).

3. Get a buy-to-let mortgage from the outset when I buy out the final 25% share even though it will cost more and I may not need it. I also wonder how it would work in practice regarding portability if we manage to sell the flat then apply for a mortgage on another property?

4. How strict are mortgage providers on not allowing you to rent out properties on standard mortgages? I'm taking it it's not worth the risk?

So confused right now