I have a mira 415 (old model) mixer shower and a new combi boiler. I am getting hot water to sinks and heating is working fine but I am getting no hot water to the shower (or hot water for a few seconds only). Any ideas what the problem might be or how I can fix this for as little money as possible?

Any help greatly appreciated!

When you turn on just the hot water do you get any water at the shower. Check and see if there is a separate valve to shut off the hot and cold water,mite be the vale is just closed on the hot water valve feeding the shower.You can only tell by running the hot water vale in the shower.If you have water on the hot side valve it should be hot in therory.Also has any one done any plumbing lately in the house,they mite have connected the hot and cold water lines to just the cold side.Ive seen that be4,lol.. let me no how you make out so if you need more help ill try a few more solutions.

Your mixer could be plugged on the hot inlet side. If you know how shutnoff the supply lines open the valve then remove the handle, escutcheon plate and remove the cartridge. Clean, grease the valve body and reinstall.

I had British Gas out to check the boiler as it had recently been replaced. They said it was definitely a problem with the shower but since they left I have noticed the following:

If I turn the hot taps on and leave them to run for a while I then get hot water to the shower but it only lasts for a few minutes then I have to run hot taps again to get more hot water.

Do you think this is the cartridge or the boiler?

If you have good hot water elsewhere but the shower I think its your mixing valve, see post #3, try it.