My husband has just come home and he's had a go at it too. It's so stubborn that we think the pedal has been put on cross-threaded.

When the LBS put on the pedals (it was the female assistant) I said to her a couple of times that I thought it was going on in the wrong thread. She tried putting the pedal on several times before it went on relatively easily but I had my doubts. Looks like I was right and it was put on cross-threaded. If this is the case I will probably need a new crank on that side and as far as I'm concerned the bike shop can replace it.