So if the house was worth £80,000 in total, the government would loan you up to 20% which would be £16,000, you would put down a 5% deposit of £4,000 and you would take out a 75% LTV mortgage of £60,000. The benefit to this is you own the whole house, the freehold, and the mortgage rate is better than if you were buying with a 95% LTV mortgage.
The government loan is interest free for FIVE years. After that you start to pay interest on it, speak to a broker to work out how much. This does NOT start to pay back the loan.
You need to settle the loan within 25 years, you can only do this is in stair cased amounts, not a direct debit for example. So a lot of people will try to chip away at it when they come to remortgage. If in 5 years time you are able to take on the whole loan, you could mortgage it out and then have a larger mortgage.
The BUT is the loan amount is not the £16,000 you borrow, but 20% of the value of the home. So if in 5 years time your house is worth £100,000, the government's share is £20,000, they essentially take 20% of your equity profit. But the good news is, if the house drops in value, so does the government's share, so if the house is only worth £50,000 in 5 years the government's share is only £10,000, so a good time to try and mortgage it out!
For me it's a good option if you can't quite get the full mortgage now and only have a small deposit but are likely to be able to remortgage in 5-10 years. I would speak to a broker about your personal circumstances to see which is likely to fit better for you.