Thanks Wes for the detailed response
I have screwed up a bit and not left enough depth in my design for air space between the external block work and the internal fire brick wall. I will have to go with it as it is. I have also found a supplier of firebricks on EBay at a reasonable price, i will chance my luck with a strong mortar mix for the fire bricks rather than a specialist mortar which seems to be expensive.
I have also found a steel supplier who can make all the metal parts such as shelves, water pans, grill and fire basket.
I just need to figure out a vent for the base of the smoker (thanks Wes for the advice of the width of the chimney in relation to the vent)
How to raise and lower the coals under the grill (i don't want to raise and lower the grill part, i would like that flush with the structure)
And finally where the hell to get doors for the smoker, i have looked at cast iron on eBay as suggested and they all need to come from Poland and are quite small.
Any suggestions and ideas always welcome and will post pics and updates as it progresses.