What is the best way to retrofit an underfloor heating system in a concrete slab?
If you have sufficient ceiling height, you could consider laying a new floor over the existing floor.

I'm assuming that since the bungalow needs refurbishing, then there is unlikely to be any (or at least sufficient) insulation under the existing slab, so you'd need to insulate under any new underfloor heating - no point having the sub-floor sucking half of the heat out of you new underfloor heating system! Unfortunately, that means that you could easily be looking at 100mm (or more) on top of the existing floor by the time you've accounted for the insulation and screed.

That in turn will give other issues to look at. Doors will become too low. Windows could need to be re-glazed with toughened glass if they then fall within the limit on distance from the floor.