It's quite easy to see the difference, without any formulas; though if you want to see the formula, google 'mortgage payment calculator' and you'll find it laid out well. Here's the approximate formula:
(Payment) = (Principal) * (Rate) * (1+Rate)^(Number of payments) /
((1+Rate)^(Number of payments)-1)
And then of course total cost-of-borrowing is Payment*(Number of payments). Remember to calculate rate as a monthly rate (look at your documents; .0323 might be your annualized compounded rate, or it might be 12*monthly rate, depends on how your local rules are). Assuming .0323 is 12*monthly rate, as it would often be in the US, then:
M = (206000)*(.0323)*(1.0323)^360 /
(1.0323^360-1)
M = 895, so 950 is a bit above that - either implying the rate is higher by a bit than you say, or there are some fees (maybe taxes?) or something else in there.
Then, at any point, you can determine the "cost" of not paying an extra 1000 pounds off by entering the details in there - make sure to fix the right number of payments remaining.
Then look at the difference. The difference in the "total amount owed", minus 1000 pounds, is the amount it would cost you to skip one extra payment. That's because what you're effectively doing is, right now, reducing the total principal.
Of course, if you're skipping that payment somewhere down the line, the cost will be less - as there's less time for that amount to generate interest - but it should give you an idea.
You can also determine an approximate answer by looking at your mortgage closing documents. Assuming the UK works roughly like the US, one thing your lender ought to tell you (in writing) is the total cost of borrowing. I.E., what the total amount of money you'll pay after 30 years. Since you may not have a fixed-rate mortgage like we do in the US, it may be approximate, but the concept should still be there.
In your case, you're going to pay a bit over 320k pounds over 30 years at that interest rate. (A bargain!) Divide 320k by 206k, and you get a ratio of 1.56. Each pound you borrow costs 1.56 pounds over the life of the loan - so reducing that amount by 1000 pounds would reduce your total cost for the life of the loan by 1560 or so (1562.80 by my reckoning).
Now, every additional 1000 pounds you pay off reduces your cost by a bit less than the first one (the first one is 1000 pounds you'd have had for 30 years, but the second one is 1000 pounds for 29.8 or so years, the third by 29.5 years or so, etc.); there's a diminishing return. But you can always recalculate the "if I had started the mortgage now" amount, and calculate it with/without the extra 1000 pounds, and know the difference of that particular skipped payment.
For example, let's say you pay 1895 per month (I am assuming my model here, assuming the extra 55 are going to taxes or something non-mortgage related), and two years in want to skip a payment. Plugging things in, each payment of 1000 pounds is saving 520 extra pounds over the life of the mortage (rather than 560 pounds as at day 1). You'll see the amount stay more or less stable for 10 years or so, then start to drop more quickly as the principal starts dropping more rapidly after around ten years (since its not linear, but exponential).
All of this assumes that you're accurately describing how the 1000 pounds is applied - to the principal. Not all localities automatically do that; I'm not in the UK so I don't know their law, but it used to be common in many places that "Extra" payments were assumed to be making an early payment of the next month, and so wouldn't be applied until then - not saving you any interest. That's slowly going away as more places realize that's very consumer unfriendly, even though it sounds "nice", so hopefully it is not true in your case: but verify with your lender that what you think should be happening with the money is.
