I've got a similar issue, although this has only been noticable for the past 3 to 4 weeks, so I guess it would be something wrong with the tap as pressure is fine on all other hot taps in the house, and yes we have a cold water tank in the loft.
I've got a similar problem. The flow of hot water is very slow using the bath mixer tab. However, haven't done much plumbling before (read zero), so any guesses what it could be ? Maybe a washer ? Is there any easy faq on dismantling and reassembling taps, on the net ?
The flow of hot water...