Gabor,
PM received and replied to.
The tubes are fairly easy to remove, however you will damage them in the process.
They are a tight fit in the head and also will have had a coat of locking sealant on assembly.
They are only pressed into the head by 20mm.
Grab the tube with a set of mole grips and move it in a circular motion to break the seal.
It does not matter if you butcher the old tube as it will compress with the use of the grips.
Just keep turning the tube back and forth whilst working it upwards.
Make sure that you keep it square and a slight tap on the mole grips with a small hammer will help it on its way.
Once you have done the first one, you will wonder what all the fuss was about and the remaining 5 will be child's play.
Alex.