Oh no, not ANOTHER cold shower?

Sorry if this is repetition, but I want to make sure I have tested everything I can, before ordering a new part.

Thermostatic bar shower (Bristan) on a conventional boiler / vented system. Fed via a pump in the loft.
Shower has been getting lukewarm at best recently.

Last night I turned off the cold supply to the pump and ran the shower- pure hot came out, so I know that hot supply is ok.

I've removed the bar and taken the cartridge out- I assume that this is the likely cause, but wanted to check if there is anything else I could eliminate. It doesn't look too scaled up, despite being about 9 years old and living in a very hard water area. Are there any visual signs that it's knackered?

I've read that the bar could be back to front, but we've definitely had hot showers out of this in the past.

You can put the bar on back to front but this is not likely especially as it has been working. It's a similar setup to mine except my pump is in a cupboard in the bathroom. It is, almost, certainly the cartridge. Cartridges tend to be expensive even assuming you can find the right one. I tend buy complete shower bars off Ebay. They are cheap, around £35 or so. These last around 5 or 6 years before the cartridge packs up then I buy another. You can of course buy another Bristan bar. The fittings should be standard. Isolate the shower if you can, I have ball valves in the hot and cold pipes on mine, unscrew the two large nuts that hold the bar to the feed pipes and fit the new one. Dead simple, or at least that is the way I do it and mine is dead simple.

I have ordered a new cartridge - £34.99 from Screwfix. I think it must be a popular model/range keeping demand going. I will no doubt be back if it doesnt work. My only doubt was that the valve allowed the pure hot through when the cold was isolated.

Yes I have no idea how the cartridges work, it's all witchcraft. But for the price of a cart I can buy a brand new shower bar off Ebay although not a Bristane I will admit. The fact that hot comes through when you turn off the cold I would think is par for the course. The cartridge regulates the temperature from cold to hot. It's never stone cold and never too hot, or at least mine isn't. Thats the magical mystery bit for me, so if I turned my cold off I would expect the water to be hot no matter where the control is and if I turn the hot off then I expect the water to be cold no matter where the control is. But as I say these showers are magical mystery things to me I just replace them.