Trade site charging VAT if customer does not provide an EU Vat Number

[irishshopper]

www.mldcrafts.com

We are setting up a TRADE ONLY site with prices quoted exclusive of VAT.

For customers outside EU we do not add on the UK 20% VAT.

For customers who are in UK or rest of EU we do not add on VAT if they quote their VAT number on registering their account with the site.

For customers in the UK or rest of EU who do not quote a VAT Number we add 20% VAT onto order total.

How can we add on an extra field on the registration asking for their VAT number and how can we achieve above ?

Are any add ons available ?