Thanks for all your replies. To be fair to them I misread their note that said it will take 2 to 4 weeks etc. to mean completion of the entire process, not thinking that it might take 4 weeks or more for the package to cross the Atlantic Ocean and then the USA. But on re-reading I can see that they were referring to exactly that part of the journey.
It has now been slightly over 4 weeks and it still hasn't arrived today. When I contacted them they replied quickly and asked whether I'd like a new kit to be sent, so I agreed. Then today they sent another email saying that my query had been referred upwards and again offering to send out a new kit, if my original one hadn't arrived within the next 7 days! A bit of a left hand, right hand problem!
As Gadget commented, I have also received the warning email from them today about the potential security breach, but I had already changed my password and turned on 2-factor authentication.