I think a good way of making that £70 billion nuclear legacy clean-up figure look a lot better for your Greenie friends is to compare it with other clean-up costs. I don't personally know any Greenies myself. We don't get too many of them in the North of England as David Cameron found out when his "Vote Blue Go Green" local election campaign floundered up here.
The £70 billion is currently intended to be spread out over 135 years which makes it an average clean-up cost of £520 million per year.
Now that doesn't look bad if you compare it with the cost of UK chewing gum clean-up which is currently £150 million per year (reference: http://www.tvlsn.org/tvls/index.php?p=news.tem&rn=anej6maa )
or UK graffiti clean-up which is currently £1 billion per year (reference:
http://www.encams.org/campaigns/main2.asp?pageid=34 ) I couldn't find a figure for UK asbestos clean-up but I would expect it to be at least £ several billion per year.