Two ways to handle it
The shares stay fixed
Whoever pays extra does so by choice. It doesn't change who owns what, and nothing is owed back. This is the simplest option and there's nothing to keep track of. It suits couples who see the mortgage as part of running a shared household, like the food shop.
The extra is paid back
Anything one of you pays above your half is owed back to you, usually out of the other person's share when you sell. It's fairer if your incomes are very different, but you'll need records of who paid what, every month, for years.
Neither is right or wrong. What matters is choosing one and writing it down.
Lump sums are different
A £30,000 overpayment from an inheritance or a bonus is a bigger deal than paying an extra £100 a month. Make sure your declaration says whether lump sums count. If it doesn't, and one of you makes a large payment, sign a new declaration that records it.
Missed payments
If one of you can't pay their half for a while and the other covers it, that's different from choosing to pay more. A good declaration says the person who covered the payment is owed it back. Otherwise the one who stopped paying effectively gets a free ride.
What the lender sees
None of this changes your mortgage. The lender treats you both as responsible for the whole loan. If the payment isn't made, it can chase either of you for all of it. Your declaration only decides what you owe each other.
If one of you moves out
Decide whether the one who leaves keeps paying their half, or whether the one who stays takes over the payments in return for living there alone. Either is reasonable. Leaving it open is how disputes start.