Sd620f problem with shower?

What type of boiler is the sd620f , I have one in my flat and it is boxed in, I think the vent must be in the loft space and then goes out.
Does that mean there is a tank and pump up there as well?
My shower has never been hot, and I have to reduce the flow rate to have a warmish shower, with not much power, what is the problem?

Zahra
November 2011