mortgage interest again

Yes. Don't tell me you want to know how to work out x. Because then I'd have to drone on and on while everybody buries their heads in their hands thinking "not again!".

None whatsoever, unless you get a change of rules from one which doesn't to one which does allow you to make unscheduled repayments whenever you like, and has them credited instantly to the account instead of at the end of their accounting year.

If you remortgage (which means change lender) to the same term end date, with the same interest rate, the payment size, and interest paid, will remain the same.

If you want to reduce the term, you will increase your payments, e.g. as you said they might go up from £850 to £1400, and initially the £850 might be £600 interest and £250 capital, so then the interest will initially also be £600 (and hence the capital £800), so the debt will shrink faster (and hence the interest part of the payment will reduce faster). Also, since you will make fewer payments, the total amount of interest you pay over the life of the loan will be much less.

It's worth remembering that mortgage interest is often calculated at the year end only, unlike most other borrowings. It depends on the bank/building society, but it would not surprise me if they gave you the same credit for paying an extra 6000 at the end of the year as they would if you paid an extra 500 each month, provided you made the required payments each month. Check with your lender.

Certainly this is the case with one of my Halifax mortgages.

Andy