Thanks for all of the above. So is it a fair assumption that all of the combi boilers have much the much same components and therefore quality of components?
I wouldn't go much further than assuming that they will be of satisfactory quality and fit for their intended purpose. As a consumer, your best guide is probably the warranty.
If you want a figure of merit to inform your the decision, use the annual cost of ownership. Take the total cost (boiler+installation cost for boiler+extended warranty) add the servicing costs for the warranty period then divide by the number of years of warranty cover.
I think it's a good idea to have the same boiler that the installer (who I also use to service it) recommends and has in their own home. This means that they will be intimately familiar with its quirks.