Vauxhall Vectra C 2.2 direct petrol - timing belt? - juliaann

Intermittent problem with loosing power which appered to go after service three weeks ago including engine flush as mechanic said it hadnt been serviced for a long time. We have only had it a few months and was told it had just been done. After service it ran great but came to use it on 28/12/14 and it wouldnt start. Green flag came out to it couldnt get it goung so towed it home as no one open to look at it. On monday aa came out to it he said it was flooded and got it going I took it to garage who checked it over and put it on diagnostics but all clear had it back tues. It then ran great until thurs when i came out of a shop it wouldnt start even though it run great 10 mins before. Aa couldnt get it goung and took it to garage. Nothing again on diagnostics. No warning lights or messages on dash. They are saying its the timing chain. Not broken but stretched.

Why would it have run so well in the days between. Are they right???

 

Value my car