Yes, act quickly!
It is irrelevant how much it cost - they've done something which significantly alters the kerb appeal and thus value of your house.
These are not your friends... Friends don't act like this... They are taking advantage of your niceness... You sound lovely... This is what they're playing on. That you'll just put up, and shut up, while they massively increase the value of their property, while devaluing yours.
I'm looking to buy now. It would seriously put us off if there was a shared access drive, and the neighbours had already essentially stolen part of the land... We'd be worried that these neighbours would continue to be a problem for us. Massive red flag!!
Your niceness (and your neighbours' under handedness) may cost you your sale, or a much reduced offer.
You need to act now, so there is NO DOUBT that this is unacceptable! If you leave it too long, they may argue that you've accepted it!
Please get some proper legal advice. Actually I would say it's so potentially serious - I would see if I could get some proper advice from a property barrister, it may save you in the long run, rather than CAB, they can talk you through all the potential issues and find a way forward.
Many barristers are now direct access (the public can see them directly, not via a solicitor). A property barrister will have all the arguments you need in their head.
Many will give an initial consultation free of charge.
You do want your neighbours' actions to stand in the way of your future.
I would guess that any legal advice would suggest that you send a letter to the neighbours outlining your position. So they are in no doubt ab9ut the unacceptability of the work.
Something like:
Dear neighbours,
we agreed this on the 5th of X. We agreed the drive would be blended in, as you wished to pave your drive. We agreed to this, but indicated we would be unable to pay for this.
On our return from holiday, we were horrified to note that your builders had come a week early, and that in OUR absense, had carried out your instructions on our land, in such a way, that it now gives the obvious impression to an impartial, reasonable, observer that no 9 owns all of drive... A substantial part of our property.
We note that this substantially improves the kerb appeal of YOUR house, while being detrimental to our home.
We want this ot be resolved at your cost.
Please indicate when this work will be undertaken.
Yours
Pissed off friends.