Unfortunately didn't realise there was Selleys spakfilla stuck on the carpet which I presume has been there for over a month. I tried removing with hot water, using scissors to break it up. Doesn't seem to work.
Spakfilla is a cement/plaster and should eventually break down with water and agitation. I'd suggest continuing to apply warm water and a little dishwashing detergent to the area. Try using a pin or fine comb and running it through the carpet fibres to break the clumps up. The only way I'm aware of to remove a cement-based product would be by tediously picking it out of the carpet by hand.
You might find that this is a mark that you ultimately won't be able to remove from the carpet. Perhaps you could enquire whether a professional carpet cleaning service would be capable of removing the material.