Two differnt meanings of ''collapse''
1) The Euro/Sterling exchange rate collapses in favour of sterling. 90% of us probably bought at an exchange rate of 1.40 to 1.50 then had to kit the place out at 1.20, it's then fall back to just above parity before recovering to its current level. The 'proper' trading range (neutral for the sterling and eurozones) would be a trdaing range of 1.20 to 1.40. 1 to 1.15 is just as false as when it was over 1.50.
2) Countries start leaving the Euro e.g. Greece. They have to have a starting exchange rate for their new drachma currency against the euro, sterling, dollar etc (like a new coy floating on the stock exchange). If they start it too high (false rate to keep asset/debt values up to please their creditors then the euro would in all likeliness strengthen against the drachma when market forces take hold. The reason the Euro is flawed is Greece, Spain, Italy, Portugal et al cannot cope with what is a fixed exchange rate against nations such as Germany. The cost of Greece leaving the euro on their own (they account for just 2% of Eurozone GDP) has been estimated at 220e billion to the other European states as Greece's euro assets and liabilities would be re-valued downwards - this would hurt, but lets face it Greece hasn't a cat in hells chance of paying back what they already owe.
Jon, agree with you apart from 2 things: 1) the 'correct' rate for sterling/euro - but that's an opinion and 2) for all those of us who have bought property in Spain, then if Spain left the eurothen those who have a substantial mortgage will probably do well as debt gets revalued downwards as well as the property, but for those who bought in cash then they will lose a packet in asset value (sterling) if they sold and looked to convert back to sterling, but yes if your income is in sterling then you'll suddenly get more bang for your buck in Spain/Greece etc.
Personally i think Greece will exit the euro but Spain won't coz if Spain leave then the euro is dead.