Its position also helps: placed against a warm house wall, in a heavily shaded but relatively sheltered part of the garden. I also gave it a light prune in mid summer, to reduce its size, so that it would less likely to tip over during winter gales.
I have also been feeding it regularly. Earlier in the season I used a domestic product for ericaceous plants. More recently I've dosed it with Solufeed, a specialist powder that you mix with water, to treat lime-induced chlorosis. This treatment - and position - seems to have worked well. After its flowered, I'll repot it with fresh ericaceous compost and some controlled-release fertiliser granules so that it does well next year too.
The latter I initially positioned incorrectly, in a well-lit but cool room, and they did next to nothing for several months. They actually need warmth as well as light. Once I realised this, and brought them into a warmer room, they started into growth. They have now been flowering for three months, on successive stalks. A big, showy bulb, to be sure, but they did the job and brightened up my winter. I shall be practising my propagation skills on them once they've finally finished flowering.