Invoicing /tax query

[Mr_Z]

For reasons in addition to the impending standard Gordon B clampdown I have to leave my umbrella co and form a ltd. I am forming the company this week, and will be billing my agency for March work on 1st April (they only pay monthly). Hiowever, It's just occurred to me that I'll be billing for some work done before my company was formed (from 1st March until now)

The agency are fine about sending me a new contract backdated to 1st March but are there any legal/tax/logistical difficulties anyone can see with this? It's imperitive that i don't use the umbrella for my March salary....

thanks in advance

[Bluebird]

the only other one I think you need to take account of is the VAT registration date - this can be backdated as well...