I'm a commercial and domestic time served HTG ENG and its gotta be the worcester bosch for me-every time 16 years in the game,if you want a really good combi boiler without breaking the bank go for a 'remeha' avanta plus 28c -really good boiler for the money,5 year warranty also.
Main eco 25's and 30's are also good,top VFM also from these boilers
I've had nothing but bad experiences with valliants over the years,but that's just personal experience