I have looked at their website, and they seem very keen to sign people up to sell on their card reader products. It looks to me like an organisation which is set up more to acquire selling agents than actually to deliver what they sell.
Their office address is SumUp Payments Limited, 32 - 34 Great Marlborough St, W1F 7JB, London, UK
ID is Company ID: 0783 6562. Opengov UK shows the registered facts and directors at opengovuk.com/company/07836562 It includes a list names and addresses of Company Officers, and one of Persons with Significant Control (which, apart from the Company Director and the Secretary, consists of corporate entities in Antigua, Barbados, Cayman, Switzerland and Luxemburg)
Letters direct to the Company Officers may do some good - or may not!