It seems the new Island Building Tasks have deleted all my progress on the other tasks I was on with.

I was close to finishing 3,2,1 Boom and another task (the name of which I've forgotten) but now they are nowhere to be seen...

Same here, the other tasks are gone

Edit: Once you go past the first three new tasks the old ones appear! My progress was there :P

Hmmmm...but that means having to spend a fair few thousands coins...

complete all the builder tasks first :) and the others will be back

As StepS said, they should come back once you complete the Builder tasks.