They take EUR there (have some left over) or do I need to change into the local shekle....?
You want the local money - kunas. I had some Euros too, and they're useful if you take a trip to Montenegro (they're not in the Eurozone, but they use Euros!). Didn't try spending them in Croatia though, and never saw anything priced in anything but Kuna.
You will find plenty of bureaux de change that will exchange your Euros for Kuna. Tourist shops/restaurants occasionally take them at a really rubbish exchange rate. But it's not like Montenegro where Euros are legal tender and it's not like - I don't know where any more actually - where people use dollars/Euros among themselves on a daily basis.
get rid of those euros as soon as possible, i say.
It's pretty pointless changing Euros for Kuna though. You pay once to convert GBP to Euro, then again to convert Euro to Kuna.
Surely it's just easier & cheaper to stick the Euros in a tin for the next time you go to the Eurozone?