PaulM,
I would agree with mrbungalow, drop the cavendish! I took a pup from mine and it grew better than the parent, possibly becasue I grew it in good compost, but once it gets to about a foot tall it seems to stop. The parent plant made two new growths from the one stem after last winter, it doesn't collapse, it just doesn't grow. Plenty of pups on both but I see no sense in trying with it any more, I decided to bury the parent plant along with it's pups in the ground this autumn, mulched well and leave it to fate. The other one is in the greenhouse, I don't think I will bring it inside, if it survives I will bury it somewhere too!
Williams Hybrid on the other hand is well worth growing, it is used for commercially grown bananas. I have high hopes of getting nanas! It's very attractive too, after last winter inside I found the rear side of the stem that didn't get the light was bright violet. It was stood on my kitchen/dining room floor somewhere near the south facing window but didn't get much good light, it's tough. The leaves when inside have red markings, but outside they seemed to bleach out perhaps with the heat this year. I was worried it wouldn't survive when I got it, it was so tiny and in December, easytropicals sell them on ebay but I don't know if they have this now. They do have orinoco at £3.99 for a plug, but postage at £2.99 can make it expensive if it doesn't live. I just potted mine up and left it moist in the bedroom and it got through. I also bought some Alocasias and Colocasia, one died, one has grown well, the other is Alocasia Purpley and it just started to reroot this autumn after a dormant spell, started to shoot and stopped but is developing a stronger crown, they don't make proper bulbs but it's alive! Plugs of those are very risky, the one that died had made a tiny bulb less than 1/8" diameter, a tiny watering in spring rotted it! If they are selling these on ebay the postage should be less, I have seen people paying a fortune for some of their plugs when they could have bought them with 'buy it now', they just bid and don't look! I think many expect to get a big plant like they show in the picture!
http://www.easytropicals.com/pcp/Bananas.html