Mercian measurement query

I began to get worried I had got my dimensions wrong
Don't you mean they? If that's the case then tell them you want the frame rebuilt to the correct specification. By remeasuring you and telling you a different size, they admited measuring you wrong in the first place.

Don't ask them for compensation for your time spent. Don't ask them for a reduction in price for a wrongly sized frame. Their frames are all lugged (right?) so it's not even that big of a deal for them to redo it. They've probably got loads of time on their hands due to the recession anyway.