So really this is a setup promotional post.
It appears I have hit a raw spot. Read back on the thread, it is obvious I am looking for the best solution to a problem. Navien, up to now are the only maker to offer a solution, Baxi may come into it after I call them tomorrow. Navien do not offer the one I want for now, which is an inside combi and extended flue.
You would do well to get Vokera to have a twin flue system like Navien. Makers do need feedback.
As far as I am aware there are makers who offer twin flue solutions, Keston do, but that boiler being so bad is rejected, ATAG do and I believe Viessmann - both on the expensive end of the scale. So far, Navien stand out in this respect - well not just yet as their new flues are not here yet. So the fallback is outside installation, which again they stand out, although two other makers to my knowledge do supply outdoor cases for their boilers, inc Vokera. Vokera being nearer the budget end of the market, is cheap enough even with an additional external enclosure, however the enclosure needs a canopy over it. However space precludes an external enclosure. My first choice was an Intergas, but the cheaper Rapid cannot have a twin flue, the more expensive other models can, then their twin flue system is lacking, of which I have explained.
I do not work for any manufacturer, being very much independent on the installation side. I just want good products that are well priced. But I do admire some Navien and Intergas products.