Tool to remove deck drain?

[NormanS]

I am sure if you know the AF size there will be a local HGV repair shop that would lend you a box spanner - they use them fro adjusting wheel bearings etc. No need to fork out 50 beer tokens for a one off job

And you'll still need something to tighten the new nuts with.

 

[rosssavage]

I've found the best tool so far is a rolson adjustable oil filter remover. It's just not quite man enough to undo the corroded on existing nuts, but will be fine to tighten new ones I reckon.

 

[Daydream believer]

They do make a pair of mole grips with a chain fitted to the jaws
If you can get in close enough & to the side then you might be able to use one.
Handy for removing filters afterwards

Check the size across the flats - long shot but it might be the size of an emersion heater spanner or similar which you can get from a builders merchant, B & Q etc