I will have to wait & see if anyone is breaking an XT 250 & hopefully get a cheap CDI as they are extortionate prices on Ebay etc. At least i know now what is wrong, thanks for all the help.

I guess to a certain extent it depends how much you're into the bike for so far but, if not too much, then it might be worth spending £300 to £400 on a brand new box (fleecebay shows 1 in Germany and 1 in the US).  

Or maybe this one?  www.regulatorrectifier.com/products/1980...amaha-xt250-cdi-unit

Doesn't the CDI box get some kind of changing signal from the pulser/source coils to determine the RPM to apply advance? Perhaps a check of those coils first might discover a less expensive fix?

Fair point.

Any advice on how to test them . Regards

Engine is revving up fine, timing is not advancing.  If CDI didn't get increased signal frequency from stator indicating that RPM was increasing, how would it rev up?   

There is a hack method of "repairing" CDI boxes that sometimes works.  I have a friend that has done it twice.  But there is a risk of destroying the CDI completely as well.  Here's a random video on the topic. 

When the strobe was on it at idle the F was on the mark on engine casing as i revved it up the speed increased but the mark did not move & the strobe was constant and was not intermittent. Hope that makes sense.

Yes, makes perfect sense.  Which is why I find it highly doubtful it's any of the stator components.  Sounds 100% like CDI box is stuck on full retard.