Strange problem with hot water

[bino]

Hello all
First of all this site is excellent.
Now for my problem, I recently fitted a steam shower cubicle which i bought on eBay it has a mixer shower not a thermostatic one. My problem is when I run the hot through the shower it warms up but then shuts off leaving me with cold water coming from the hot.
If I have the hot tap in the bathroom on just a little the boiler works fine and I get a steady supply of hot in the shower.
I had Brittish gas out yesterday to chek the boiler and they seem to think it is fine.
My boiler is a Worcester Greenstar 24i junior
any suggestions most welcome

many thanks bino

 

[seco services]

is it combi compatable ?
thermostatic will work better.

 

[bino]

shower says it is compatible with combi boiler it is as if there is to much pressure when just running the shower as as soon as i turn on the hot tap in the bathroom it works fine