I have to say that Fulvia heaters are proverbially feeble. Good luck anyway!
I must have been lucky with all mine over the years (1x S1 4 x S2) then!! I always give them a good reverse flush, make sure the tap is working / not bloked with sealant, and use 50/50 blue antifreeze to limit corrosion. S1 is not as effective as S1 as my current first series Flavia shows. But it still provides heat. We are all used to modern cars able to heat a cabin up in 300metres of driving from cold....