Diverter valve or shower mixer?

Hello
I need help because my Homecare plumber talks to me as though I am a girl who knows nothing. And I am!

Problem: My shower is only giving us lukewarm water, yet the water from the bathroom sink and kitchen sinks runs hot, cold, lukewarm, whatever we want.

I suspect the problem is just the shower mixer but the Homecare engineer we called out is coming back with a replacement diverter valve...if he can find it. As he suggested the part may not be available any more due to the age of the boiler. He then said that it was an old boiler and that if I did need to fix it, he could do it cheaper than British Gas.

I am now paranoid that he is going to do something bad to my boiler so then i have to replace it...maybe he is hoping to get the job given that he said he was cheaper.

Can someone offer some good advice? Surely I wouldn't get hot water in all sinks if there was truly a problem with the diverter?

many thanks

If the other taps are giving hot water on full flow its most likely the thermostatic cartridge in your shower.

if you don't trust bg, why do you have a service contract with them?

i am not advocating you should by the way, it just seems to be a bit of cart before the horse

my opinion has always been that you are far better off with a trusted, local engineer who knows you, your boiler and your needs

i agree its more likely to be the cartridge in the shower.