Unfortunately the area where the shower will be has already been plastered and tiled. So won't be able to pull through.
Did he come and inspect the installation of the cable before it was covered over?
But what is correctly? What sort of connector?
Your electrician is the only person who can legitimately answer that for you.
The reason I'm asking these questions, and making these points, is that this is notifiable work - it needs Building Regulations approval.
If you do it, then you needed to apply for approval before you start, which would have involved you explaining how you would comply with all the relevant Building Regulations. I'm guessing that you didn't take this route.
So what's left is using a registered electrician who will notify the work after the event via his registration scheme. The law requires him to declare that the work he did complied with the Building Regulations. So is he prepared to say that when he didn't actually do it all?
Plus, best practice and the requirements of his scheme membership mean that he has to sign an Electrical Installation Certificate:
I being the person responsible for the design, construction, inspection & testing of the electrical installation (as indicated by my signature below), particulars of which are described above, having exercised reasonable skill and care when carrying out the design, construction, inspection & testing hereby CERTIFY that the said work for which I have been responsible is to the best of my knowledge and belief in accordance with BS 7671:2008, amended to 2015 except for the departures, if any, detailed as follows:
Here the parts I've highlighted are an issue. Choosing the cable type and size is Design. Choosing the route is Design. Choosing any junction box is Design. Being "responsible" for construction means, at least, telling you exactly where and how to install the cable, and supervising and checking that you did it as instructed (hence "
Did he come and inspect the installation of the cable before it was covered over?").
Is he prepared to sign an EIC?