Question about potatoes

I've got two potato bags with (second earlies?) in, i.e. Christmas spuds. They were moved from the garden into the greenhouse at the start of October before the temps dropped too much. I'm watering them every 4 days or so - when the compost feels like it's starting to dry out (not bone dry, obviously). The foliage is starting to thin out and many of the lower leaves are turning yellow. I'm unsure what to do next. Should I keep going as I am, and wait until the foliage dies off completely before having a rummage for spuds? And if so, would there be any benefit in giving a comfrey feed at this stage to keep the compost nutritious? Thanks in advance.

 

Thanks very much @JWK!