I've looked through a few threads, but still not really sure where to go with this.

The hot water in the shower seems to be of quite low pressure... it eventually turns scalding hot, but as soon as any cold comes on it seems to overpower it and turn it off. Occasionally I can get the balance just right, but it's very awkward, and will slowly change towards cold or hot.

The shower is relatively old, and controlled by individual hot and cold knobs. The water heater is in the roof of the unit, therefore a bit hard to get to, but can be accessed if necessary.

Hot water seems to come out fine from all other taps - maybe a bit low pressured though, but seems to mix fine at the sink.