This steriliser holds 4 of the 250ml bottles and comes with 2x250ml bottles included. There's a third off the bottles too I think the large ones are around £4 for 2.
I just bought this in store from Trafford Park and there were a few one the shelf. Available on-line too.
From the website:
Boots Microwave Steriliser uses steam sterilisation which is the best way to protect your baby from harmful germs. This unit uses the speed and convenience of your microwave to ensure your accessories are safe for your baby. The unit has a built in steam release vent which will allow excess steam to escape during the cycle ensuring that it is safer for you to open when complete.
Dimensions: 260mm x 260mm x 183mm
Stylish compact design ideal for travel
Simple to load easy to use
Fast and safe sterilisation
Holds four wide-necked bottles
Suitable for sterilising breast pump and accessories