Three hours was no issue, even though I could have changed the PSU in ten minutes (I could also have repaired it but that would have meant breaking the seal and voided the warranty). But the chap behind the counter was a one man band on the day - with a shop full of customers waiting behind me. I wasn't so desperate to insist he stopped everything to do it while I waited! I went off and did something else for the morning.
New PC works very well now.