Following on from Glen's instructions above, it may be that the Gaggia Classic is a bit simpler than some but I found that I could skip a lot of the steps. I of course started on some of them and then found that they were superfluous for my particular evolution of the machine. It did help that I have a tiny Topeak ratchet spanner for my bike that allowed me to do the 4mm bolts with very little space.
Firstly make sure the machine is turned off and cold (OK so I was impatient and burnt myself a bit)
Remove the 2 screws holding the funnel and lid then lift off and disconnect the earth wire YES
You will see the boiler, the solenoid is on the left hand side (EASIER TO say that it is on the opposite side to the steam knob at the back of the boiler)
Pull the steam knob off and using a 17mm spanner untighten the steam arm NO
Using a 12mm spanner undo the pipe connection to the boiler. It can be done without removing the boiler terminals but I would recommend marking the four top ones 1 to 4 with black marker just in case NO
The boiler unit is now free to lift out. NO
Use 4mm allen key to remove the 4 allen bolts holding the brew head and lift out. NO
I REMOVED THE little pipe that comes out of the top of the solenoid at this point
Remove the 2 wires from the solenoid and using the 4 mm allen key remove the 2 bolts holding it on. YES - found it easiest with the machine sitting on its front so that the spanner was swinging in horizontal plane.
Remove the top bolt from the valve and lift the coil (black box) off. YES
Now you need to remove the tower, a vice is recommended but it can be done with mole grips as I do. THIS WAS easy with fingers and a spanner - depends on who assembled it !
Remove the 2 seals to avoid damage, these can be re used. AT MY RISK I left them in to avoid damaging them whilst removing. They seemed quite well embedded.
Then grip the base and using spanner, can be 17 to 24 mm depending on type of solenoid then remove. AGAIN, AT my risk (and you have to make your own choice) did it with spanner and fingers.
The centre hole is one that will be blocked use a paper clip or something similar to clear the blockage or soak over night in descaler if badly blocked, run under tap into the right hand inlet and make sure that water comes out the centre hole. I USED a strand from a plastic washing up brush and then stood the assembly in vinegar for an hour, dropping vinegar into the two large holes to run through occasionally.
Then replace by following the reverse process. AS REQUIRED
Coffee now coming through perfectly so thank you very much for the instruction about where the problem was. Unfortunately I now have no excuse to buy a bigger and better machine :-)