Hi I've checked the flue and it's fine really sturdy I've braced it with metal straps so it's going no were it did have a service last year the installer checked it first service free so then took out the extra warranty with baxi I've spoke to the lad who installed it and he said as the heateam bloke as said it is not to current standard it means nothing
It's still "at risk" with flexible strapping, it has to be supported with fixed rigid brackets.
Unless I could confirm the plastic pipe used for pressure relief was suitable and a HEPVo trap was used i'd NCS it too.
Sounds like it wasn't installed quite right, the heateam guy is just going by the book I think.