Has anyone used nematodes?
If so where from and how effective.
I've found them good for potatoes in the past but haven't used for years but think I will have to this year.

We use nemaslug on our potatoes.  It does a good job !

We find it helps as a preventative, but its not a silver bullet.  Spray with aspirin when the forecast Hutton Criteria for the area are red (see https://blightspy.huttonltd.com/#/forecast) with the idea that it stresses the plant, closing the pores and therefore making it less vulnerable to the late blight spores.  Don't overcook it and weaken the plant, keep them well fed and strong, and take off excess foliage.  All together it increases the odds of a good year!
Once they have blight, you can try removing the affected foliage, but you are only buying a bit of time.    And on an allotment site, once one plot has it ... again it is only a matter of time. 

I do like good value but my son says buy once but well.

 A bit better today 4 

I'm gong to  have a go at growing Erysium from seed a variety called Bowles Mauve has anyone on this site grew Erysium from seed and if you have could you give me some advise that would be most appreciated RW