Changing capacitor C3 on the main board of my Opal-2 has fixed the problem for me. A replacement capacitor is available from Maplin for less than £1. Make sure it is rated 'X2' (as Nigel S says). The capacitor I got was marked: 0.1uF (microfarad), 275v, X2. Thanks go to Maplin for stocking such an item in a high-street store.
Many, many thanks to Nigel S for finding the solution for me at long last.
For those who might want more technical info for diagnostics, the power supply to the two 4025 CMOS ICs on the push-button board was fluctuating just above and below 3v on pin14 (with the 2 ICs at the bottom, pin14 is the top-left pin of the IC) - the voltage should be nearer 7v to 8v. The fluctuating voltage causes the ICs to behave erratically, because their minimum working voltage is 3v. Because of the instability of the power supply, some of the transistors behaved erratically and became temperature-sensitive (when T6 was warmed up, the shower started to work again).
The pressure switch on my Opal-2 was fine, but the same old story as for many others - the 'low pressure' warning light kept coming on and the shower went cold. In my ignorance, I changed all the semiconductors first (ICs, transistors and diodes) because I thought they would be more likely to fail than resistors or capacitors. Oh well, an electronics lesson for me.
WARNING! There are dangerous voltages inside the shower than can kill you. Switch off the shower at the fuse box, and at the bathroom switch and make sure that it is well and truely dead before you open it. Replacing the capacitor C3 requires the ability to use a soldering iron to make a reliable solder joint. A poorly installed component (or a wrongly rated one) could be dangerous or a fire risk.
Malc, November 2010