Allocating activity according to date.

I've attached a sample workbook with before and after examples, which will no doubt be clearer than my explanation here.

But this is the problem I'm asking for help with.

Blocks 1 to 20 have an input date and an output date. blocks 1 to 20 cannot accept a new input until the output date for that block has expired. Then I want to allocate the next available input date that falls after the expired output date.

See the example, that will be clearer. Be sure to examine further down the sheet to see the expected changes as the first block runs 1 to 20 then on to 26 in the second block.

Thanks for looking,
Regards

So, no takers ?
I'd be interested to know why, is it too complicated a problem, not interesting enough or does it require too much time.

I could really do with some help with this, if you need more info please just ask.