Can I suggest you adopt the 'square foot' gardening method and plan your small plot on the basis of one plant per square? If you look up Mel Bartholomew and his 'Square Foot Gardening' technique, you will find details of how much space to use for each.
One square foot is adequate for all the brassicas you mention, except for brussels sprouts, BUT that is only if the soil is good with plenty of humus to retain moisture, and a good handful of organic fertiliser. You will also have to water frequently to ensure a good crop. I've just grown six superb caulis in the polytunnel at a square foot spacing, with daily watering. Cabbages will do fine too. If you like small tasty turnips, grow them at five to a square. The suggested spacing for radish is 16 to a square, carefully symmetrically spaced.
With the exception of radish and turnip, I usually start my brassicas in modules and then plant them out at the above spacings. Good luck, you should get a lot out of your 2 square metres.

Sorry, forgot to say that Brussels planted at the foot spacing will only produce small sprouts, but give them one and half square feet they will produce bigger ones. Given the small space you have, I'd recommend you forget the sprouts and grow winter kales instead as these will be fine in one square foot, and ultimately more productive for harvesting next spring.

Thanks Bertiefox that is something to consider. I was thinking of planting out my brassicas in rows down the length of the bed. But I am worried about overcrowding them. I m not worried about having small sprouts as the kids don't eat them anyway! haha

You can plant in rows Great but will still need a foot between rows and between plants. So you could have 6 rows each with 3 plants, or 3 rows with 6 plants.

Thanks moon grower 18 plants it is then

Yup and be aware you may have to stake your Brussels sprouts.

When do I feed my growing plants?. I have netted them to keep the birds of - pigeons seem to be a problem this year. However something has nibbled the leaves!!

Can't help on that front Great as we don't feed the plants rather we feed the soil beforehand. Slug loves brassicas as does the cabbage white butterfly caterpillars.