Unfortunately I’ve Brocken my headlight on my mk3 Focus can find second hand part on the usual sites ect but if my part number is BM51-13w030-DK what’s the difference in one with the same part number but the last letters are DH thanks in advance.

I'm pretty sure that the last two characters are down to the year of production. Hopefully someone can confirm.

The first letter of the suffix, D in this case indicates the part variant then the second letter is the engineering change level.
 

So DH is an earlier version of your spec of headlamp. Part levels progress as engineering changes are made for all sorts of reasons - quality fixes, cost downs, manufacturing changes etc. These can happen at any time and aren’t linked to specific production years. 
 

Just a word of caution, sometimes part changes are coordinated and not backwards compatible - you’d maybe see this on sites like 7 zap if they list several instances of your headlamp based on different build dates. Maybe worth checking that just to be safe unless someone can confirm they’ve swapped with no issues already.
 

Hope this helps