needs advice from a plumber

First up, my apologies for apparently usurping this site with a non related topic, but I promise to weave 4 wheel driving into it eventually.

Our Rheem 135 litre hot water system (31 series) is playing up. The water pressure has dropped off significantly and the water temperature does not seem to indicate the high setting of the thermostat (second highest setting).

This morning I filled up a 9.6 litre bucket with water from the hot tap closest to the heater (approx 1 meter) and it took 3 minutes to fill. The cold tap next to it took 13 seconds.

On cold nights we have to supplement the hot water with boiled water from the kettle when running a bath, and it takes ages to fill. Showers run out of steam very quickly also.

The problem has been obvious for some time, but we have learned to live with it, however we recently had the diode changed (by a mechanic!!), and the problem seems to have gotten worse.

Can anyone please advise what the likely cause is and a possible solution, or is that like asking how long is a piece of string?

We are planning to go off roading in the Pilbara in a couple of months (I did promise to weave 4 wheel driving in), but I can see that possibly going out the window if I call in a plumber. Is it a big problem, will it be expensive to fix?

Again my apologies to the forum readers and to the ExplorOz team, but I dont know any plumbers and this seems like a good place for my questions to get wide exposure without doing the dirty on a hard working tradesman by calling him out and then sending him packing. The unit was new in 1998.