In my humble opinion a proper valve alignment may be in order. I've had all my horns aligned by Bob Reeves in California and the results were astonishing. The most dramatic was my Bach Strad Bb trumpet. It took what I thought was a mediocre Bach and transformed it into a more open and easier playing instrument. A above the staff is now solid and in tune and high C slots in much easier. I highly recommend it.
I'm sure you have the same service available in the UK.