2002 Renault Masters engine power failure

Hi There
i just had an enigine rebuild on my renult master lwb 52 plate hitop 2.2 after being stupid and not replacing the timing belt and on the day of picking up the van from the garage when driving from glasgow to aberdeen there and going up a fairly steep hill I was being overtakin by tractors on droping a gear the noise was not to what it was before and there was a drop in rev counter butt I darent drop another gear in fear of it breaking anything this van prevouis to timing belt breaking would canter up this hill no problem I took the van back to garage and was told that its probobly not settled and needs time to adjust I respect my van as I use it for a living and do not overuse the engine as deisel is to expensive can u please let me know if there is common soloution or something not done mechanicly after paying close to £900 for engine rebuild
Thanks Kenny
p.S sorry for the book