The small expansion tank is for the open-vented boiler circuit. The big plastic ones are cold water storage tanks (CWSTs). Unusual to have 2, I assume plumbed together, but maybe to do with problem getting one bigger one in the loft. In any case, it has nothing to do with choice of boiler.
Just to close the thread - we went with the baxi boiler (10 year warrenty jobby) due to the brass fittings. The expansion tank is going, extra large water tank going, hot water tank going, boiler being moved upstairs (from under the stairs)
baxi are very good where warranty work is concerned and very rarely try and get out of work .
but breakdowns on the 6/800 range are very few and far apart
Look at warranty periods and the boiler that has best modulating ratios, Baxi/Main are decent boilers but poor modulating ratios during the era of espensive gas.
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