Manual here:
http://www.plumbcenter.co.uk/wcsstore7.00.00.549/ExtendedSitesCatalogAssetStore/images/products/AssetPush/DTP_AssetPushHighRes/std.lang.all/ti/on/Baxi_Combi105HE_Installation.pdf
Read fault analysis on Page 16
Flog through one step at a time, thinking logically.
It LOOKS like - on the basis of what I understand at the moment - that there is a problem around the bit that says "Continuity across DHW flow microswitch terminals and PCB - A5 connector terminals 5 & 6"
Read and think.
XXX
July 2016