As they are in the wring and have admitted liability I would insist on either the warranty or perhaps free servicing vouchers. But either way get something in writing.
This will not cost them much, just the labour, as the 10000 mile service is basically an oil and filter change which will cost them about £10 in parts and oil.
I would expect them to do this for you to keep your future custom and LEXUS GB happy, after all their after sales service is second to none.
You are entitled to what the salesman sold you.