Agree 22 mpg is poor unless it includes alot of urban driving and cold starts. More likely a problem with lambda sensor, split or cracked vacuum pipe, to map sensor, air leaks pre catalyst etc.
In theory EGR generally has a very small beneficial effect on fuel consumption unless too much gas is re-circulated and is causing the engine to missfire and run roughly.
Recirculating exhaust gas displaces air, reduces power output and causes unstable running so the it isn't used at very low engine speeds and I think maximum power. Would expect mostly loss of performance not increased fuel consumption if the valve had failed.
The Vauxhall EGR valve has a position indicator that should log a fault code if the valve is stuck in the open position although probably not if the valve had just got 'sluggish'