You will need to register the partnership with HMRC. They will require the names and unique tax reference of each partner. Additionally one of the partners will need to be the nominated partner. This is the partner to whom the notification to file a partnership return is sent and who is responsible for ensuring that the partnership return is filed on time.
Furthermore, although you are already submitting tax returns, you must each complete a form to notify HMRC that you are joining a partnership.
There may be penalties charged if you delay in informing HMRC of the partnership and partners. Your local TaxAssist Accountant would be happy to help you maintain your tax affairs.