You bought on the basis that your roof would have grey natural slate tiles.
This would clearly mean your roof was being tiled using actual slate, not something that looks like or is a slate colour.
As far as I am aware there is no such thing as a red slate.
I suspect they mean red flat concrete interlocking roof tiles as opposed to natural slate coloured concrete interlocking tiles which is what they should be described as.
What your neighbour wants or prefers is somewhat irrelevant. I expect he is actually indifferent and Bellway are using his view as a reason not to change the tiles to what they should have been - those that have been approved by the planners. You may even find that the planners require you to change the tiles at your cost in the future unless they are approved before you complete the purchase. You may even be unable to sell the home if a buyer's solicitor is on the ball and spots the planning breach.
You also have to wonder, given that the site management cannot even ensure the correct materials are used on the right plots, what the rest of the home will be like! At least this site manager has gone!
Mistakes happen of course and the correct course of action is that Bellway should change the roof tiles to natural slate as specified.
But not only will slates be more expensive, they will have also paid for the roof to be tiles twice.
I would advise that you insist on the correct roof tiles to be used. Yours and your neighbour's houses will be more attractive to future buyers and will be worth more with a slate roof. If, as I suspect, Bellway continue to try to fight this, you can use the Consumer Code for Home Builders to claim compensation (for refund of justifiable expenses) as they have failed to comply with the requirements, although that have at least offered for you to walk away and refund your reservation fee. They also informed you and it is clear this is a mistake and not a deliberate attempt to misdescribe.
You will also be aware, with the current Help-to-Buy stampede along with rising prices (new homes up 7% this year) that Bellway will be able to sell your home at a higher price to someone else and may even have someone in reserve!
If Bellway are not prepared to change the tiles to slate walk away.