Back to the beginning! I have, in fact, settled on the Atag. The pros & cons discussed above seem to have been mainly in respect of the various combi offerings, whereas my need was for a system boiler.
I settled on the Atag as this was the best solution for my situation, which is that the boiler is sited in a cellar, the flue route being such that neither Weissmann, nor Intergas, technical depts could suggest a suitable flue system, largely due to theirs being only of concentric configuration. Atag have provided a separate inlet and exhaust system, the inlet offering the option of drawing air from the cellar (via an inlet filter), which although well fitted with vents, tends not to have much air movement. The air consumption thus acting as forced ventilation as well.
Each of the companies' technical depts were extremely helpful, as were the comments of the various contributors on this site.
Very many thanks to you all.