Have a look for workshop dust extraction units such as this for a machine that uses 100mm hoses.
They also do a 100 to 50mm adaptor there, and before you ask; no I don't work for them!
I'm only assuming that you've got an external three-way valve; a bit more information on your system might be a help.
Which model Valiant Ecotec boiler have you got?
or
Is it a conventional or combi boiler - you can normally tell if you have a separate hot-water tank fitted somewhere?
If the system flowrate necessitates two or more then they'll all need running back to the 110mm waste pipe, otherwise you're just moving the flow restriction slightly further downstream (and under your floor presumably).
If they're only being used to aid drainage across the wetroom floor and...