In need of a thermostatic mixer shower

Is it worth spending big bucks on one or is a simple (sub-100 quid) bar mixer from a shed just as good?

bar mixers are generally cheaper than, and not as good as, round-bodied mixers.

Be sure to fit it on a surface-mounted wall plate so it will be easy to swap for a new one when it goes wrong, and you won't have to smash tiles and dig out the wall.

"Not as good" how? They dont have to do much except mix the hot and cold....

The question you want to ask is why are cheap bar mixers so cheap.

The answer to that is the thermostat cartridges are cheap crap, and made from inferior materials ...... They attest to be anti scald but I wouldn't think they would pass the TMV2/3 approval tests.

An expensive mixer will be tested and approved, made of higher quality materials and they will have full backup and spares available.

Any thoughts on Bristan? eg https://www.heatandplumb.com/acatal...ith-adjustable-riser-kit-chrome-bnpm2sqshxarc

Mid-range price , looks OK

These look very expensive, but are the worth it? https://www.heatandplumb.com/acatalog/mira-select-thermostatic-exposed-shower-1-1592-005

I've used several round-bodied Aqualisa shower mixers, after reading good reviews on them, and been very pleased. Their coiled temperature-sensor is key to the design they built their reputation on, and parts are available going back a number of years (none of mine have gone wrong) and repair (see Utube) seems fairly straightforward. They make models aimed at various price points, but as far as I can see the fundamental design is the same, and quite simple. The internal cartridge is replaceable from the front after removing the round trim and the knob. Models are available to suit combi, cylinder or pumped supplies.

Sadly they are not very cheap, but sometimes you find discontinued or special offers.

I believe this "Colt" is currently the cheapest,
https://www.plumbworld.co.uk/aqualisa-colt-concealed-64-23707
mine are the Aquavalve and look about the same.

Observe the round bodied design with the big control knob.

I think the Aspire and Siren might have the same mechanism inside, but you'd need to check the spare parts diagrams on aqualisa to check.
https://www.plumbworld.co.uk/aqualisa-showers-1265-0000