epsilondraconis

We have a 12 year old 3 bed house with two showers - both are Sirrus 1503 Thermostatic showers and both are on the same floor (one is in the main bathroom and the other is in the en-suite). Both showers were part of the original house build and haven't subsequently been added. The shower in the main bathroom works fine - when taking a shower in the main bathroom we have the temperature setting on 5 (on a range of 1-9) and everything is great. We no longer use the en-suite shower because even when the setting is on 9 (the hottest setting), the shower temperature isn't really warm enough. The thing is that if you take the en-suite shower head from its bracket and lower it whilst you're in the shower the temperature increases. If you have the temperature setting on 9, you get luke warm water when the shower head is in its wall bracket at head height and yet you get scalding water coming through if you have the shower head lower than your knees.
I've tried this in the main bathroom shower and varying the height of the shower head has no effect.
I'm currently thinking that this problem may be due to sludge in the pipework because approximately 2 years ago we had a problem with our heating system. The plumber came out and replaced the water pump that is located in our airing cupboard. He said that the pump was clogged up with sludge from the heating system. I asked him about having the system / radiators flushed and he quoted £450. Ouch. Anyway, after he replaced the water pump, we noticed that the en-suite shower was working much better and it was possible to have a shower at a reasonable temperature whilst the shower head was in it bracket on the wall (and so didn't require us to lower the shower head to increase the water temperature). However, over time, the temperature problem has returned and so we no longer use the en-suite shower.
I've just removed the wall panel that allows me to view the pipes feeding the en-suite and when the shower head is in the bracket on the wall and the shower is on 9 (the hottest setting), the water coming from the shower isn't particularly warm; however when I touch the hot water pipe feeding the shower, it is scalding. I'm obviously not a plumber, but it seems that the hot water is getting to the shower mixer unit, but for some reason, it doesn't go the final 1 metre or so to get to the shower head.
Do people think that this is a sludge in the system problem and if I pay my £450, I'll have a shower that works properly, or does this sound like a different problem.
Any help will be very much appreciated...