Flow restrictor?

[Dg8]

Hi
I was having an issue with my outside tap, where if I turned it on too far the pressure and flow would suddenly be reduced to a trickle. Replacing the tap didn't make any difference. I've found where the pipe joins the cold water feed, and just after the isolation valve, there's another inline valve (see pic). Am I right that this is some type of flow regulating value that may have failed?
I can't find a direct replacement for it at the usual haunts (Screwfix), so any suggestions would be great.
Thanks very much in advance

 

[ShaunCorbs]

It’s a non return valve

 

[Dg8]

It’s a non return valve

Brilliant thanks! I suspect it's failed somehow so will replace with a new double check one and see if fixes the problem.

 

[PETE 123]

could be part blocked as theres a small opening its not full bore.