Help to Buy - Advice needed on % of equity loan

HomelessChump

I posted a thread about this a few days ago and received some food for thought but I can't quite make a decision.

My situation is that I have received authorisation from HTB London and a mortgage offer based on using the full 40% equity loan. At the time of applying a month ago, I was just squeaking through the affordability checks for HTB. I have since received a decent pay rise and - according to the HTB London calculator - I could now afford to reduce the size of equity loan to 31%. There is time to rejig the mortgage, etc, as the exchange of contracts is still a few weeks away.

The advice that I have received so far is that much depends on my long term plans. Let's assume that I want to stay in the flat after the interest free period on the equity expires in five years - what should I do? If I go for the 31% equity loan, I'll be paying the interest on that extra 9% for five years instead of enjoying the interest free period, but the cost of purchasing that 9% is only going to go up (perhaps substantially, as the property is in a very central location).

I appreciate there may not be a clearly right course of action here and I am increasingly leaning towards just leaving the arrangements alone.

Any advice appreciated!

Edit: Just to add an additional factor, my job includes a guaranteed pay rise in 12 months. If I was on that salary today, I could afford to reduce the equity loan to 12%.

My current mortgage product is fixed for five years. Would a sensible way forward be to go for a two year product instead and, at the end of the two year period, staircase my way to 80%+ ownership?