If I remember rightly each valve is secured by being locked in by the edge of the recess being tapped over on three or four points on the lip of each non return valve. clean these and the valves should come out with slight levering they are a push interference fit.
do one at a time so as not to get them the wrong way round, one goes one way the other is reversed if that makes sense. Then just tap the rims over to “lock them in”.
Hope this helps
Also Can some of the Gurues AKA- van man or Dalek read this to correct any errors I may have made.