Can I take from the lack of response to this remark that everyone would
be quite happy with a heating system that only worked in winter when the
main house heating was on

Not sure how cold the floor will feel without any heat under it. My sister
has a slate kitchen floor onto screeded insulation, and right now this feels
refreshingly cool under bare feet. In an upstairs room it should be a whole
lot warmer. I can only suggest you get a couple of the slates out, let them
aclimatise and see how they feel.

On thing to watch is that UFH is best carried out with temperature
limited
water ... 38deg C or so as a maximum.
The water you are pumping in from your rad system might be too hot and
cause
hot spots on the floor.

Perhaps a mixing valve teed across your F & R might be worth
considering.

For small area like that it gets expensive to introduce separate zone
controls so combining with the CH is the economic option. If UFH is needed
in summer then a separate zone is the only option unless all radiators can
be shut off. A pro job would involve a separate pumped and mixed loop
controlled by a zone valve.
I see in the BES catalogue an "FJVR return temperature sensor" (p/n 15764)
plus valve body (p/n 15765). It looks just like a standard radiator TRV
except it measures the water temp and costs £25. This will restrict the
water exiting the UFH loop so that the water within the loop has the
opportunity to cool to the recommended 40deg or so. Of course a bit of
adjustment on the flow temperature will be needed, a warm floor in winter,
just take the chill off in summer.

Added to a digistat and zone valve this is the option I have selected for a
solid kitchen floor of 14m2.

If you decide you may only ever need the UFH infrequently the easier
installation of electric mat within the tile adhesive layer will win. You
should get about 125 w/m2 from the mat. I believe it's a close call for your
requirement.

Toby.