Hi all. Thanks to everyone who replied. I?ll try to cover all the points mentioned.
The machine is Bosch model no. SGS5332 GB. The dispenser is the same type as shown in the ?youtube? link given by Adam Funk. The spring is intact and the problem was that the cover would not stay closed. When I finally did get it to close, the slightest jarring caused it to open again.
There is a small nib at each corner of the cover and they run in 2 channels. I figured that as I slid the cover to the left the nibs dropped down indentations at the bottom of the channels and that is what held the cover closed. When it was time to be opened, the blue arm would rotate and by cam action would lift the nibs clear of the indentations.
So the nibs had probably worn. I tried to reform them using cyno glue, and then using Araldite, but neither of those materials withstood the hot water and detergent. Finally, I made a suitably shaped plate from aluminium and riveted it to the cover with 1mm copper wire. That has lasted for about 3months but the problem is now back. I think it is now due to wear on the indents in the channels, but I cannot get at them so I am stuck.
I had not thought about a rinse cycle until it was mentioned, so I started up the machine and stood by it. Sure enough, after about 15mins the (cold) water was pumped out and the machine refilled.
So it looks like I need a complete new unit (any suggestions as from where). The alternatives are waiting 15 minutes and dispense the tablets manually, or get a new machine (a pity because it is good otherwise).