UFH query (bathroom zone on CH return)

The abandoned bathroom project is looming on the horizon after about 15 years of remaining a stripped out cave so I'm addressing the heating dilema...
The house refurb started off with a 4 zone UFH manifold serving the upstairs living areas; downstairs bedrooms remained unheated for many years but were then furnished with room radiators on a t/stat zone valve from boiler feed. All good and working perfectly so far...
5 years ago we had a new "upstairs" extension added to the living area giving 3 more rooms (4 zones) so I fitted a 2nd UFH pump/manifold from the handy flow/return loop I'd originally left in the garage (now gym) as a "just in case" and plumbed up the new rooms with in-screed UFH, all systems still working magnificently.
I now have the downstairs bathroom left to sort out. The room is underground as the house is built into a hill so floor slab is 1970 un-insulated concrete on clay but it didn't ever have a cold problem but ideally I'd like to UFH it. The obvious way would be a single zone UFH manifold/pump but I'm running out of space in the airing cupboard although I could squeeze one in but I'm thinking I could simply use the return flow from radiators/UFH etc to do a few laps of the bathroom before going back to the boiler, possible isolating it with a zone valve on a bathroom t/stat.
Boiler has a max flow temp of 55 degrees controlled by weather compensation and return flow is around 5 to 10 degrees lower.
So... Could there be a problem fitting a 3rd pumped manifold onto the system (4 pumps including boiler pump) I've already plumbed up a flow/return spur into the bathroom for a towel rail so not getting "enough" heat from the floor isn't an issue but loading the boiler pump up with another 20m of pipe could be? My logic is that as existing manifold pumps are doing the lions share of overcoming pipe resistance the boiler pump is only serving 5 radiators and a towel rail (in the upstairs shower room) and the loop to 2nd ufh manifold so it's not excessively loaded.
Any obvious pros/cons I might have overlooked?
Cheers Pete
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