Would I be able to swap a fork from a bike with a 140mm head tube to a 145 mm headtube - assuming all else is equal, or does it have to be spot on?
Rebas from boardman hardtail in a large 140mm [url= http://www.halfords.com/webapp/wcs/stores/servlet/product_storeId_10001_catalogId_10151_productId_777721_langId_-1_categoryId_165499 ]link[/url]
to Giant trance Large 145mm [url= http://www.giant-bicycles.com/en-gb/bikes/model/trance.x.4/9335/49883/ ]link[/url]
Depends how many spacers you have under the stem as you can remove or replace for the correct size. Also if you do not have any spacers under the stem, you will be able to replace the stem for one which is 5mm smaller.
Not sure... I don't have my hands on either of the bikes. Just having ideas...
Looking at the pics again it looks like 4 under the boardman and 2 under the trance so potentially good to go!
Probably assuming the steerer length is about the same - that is what you need to know
Looking at those photos [ and assuming they have both used integrated headsets I would say a definite Yes
I swap forks around my bikes with different length headtubes - as i Note steerer length is what you need to know
also bear in mind the hiehgt on the headsets too. Mnay headtube lengths are the frame only. I'd guess giant and boardman will use some form of low stack/semi-intergrated headsets so won't affect all that much
"you will be able to replace the stem for one which is 5mm smaller"
Unless you're already on a low stack model?