low hot water pressure

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I have an oil fired Worcester Heatslave Combi boiler, about 10 years old. Serviced once a year and last service in December 2003 when engineer said all working well.
The hot water pressure has always been less than the cold, but in recent weeks this has suddenly dropped to an un-usable level. It takes 25 mintes to fill a small bath, and pressure is so bad, you can't rinse plates off in the kitchen sink! Another sympton is that the water occasionally splutters as you let it run.
Can anyone tell me it there is a simple way of boosting the pressure by "tweaking" the boiler? I seem to recall being told once, but since forgotten, that I can bleed a big red cylender thing in the boiler, and introduce more cold water to the system to build the pressure back up.
I don't think the problem is with calcified or limed up pipes, because when I fist turn on the hot water tap, the pressure is good (in fact better than it ever has been)for about 10 seconds.

Any hints or advice will be greatly appreciated!

Thanks, Antony.