Hi and good morning.
I cannot offer advice nor should you construe my comments as such. What I will do is to offer an opinion.
First over and above all I would NOT take court action. It will cost a lot of money and more than you could recover.
This is what I would do. You decide if this would be suitable for you.
1 : Obtain a survey of what needs to be done to make the installation compliant. This would be best obtained from the housing association, who I hope would be helpful to you in the circumstances. If not pay for a survey by a competent electrical contractor who should set out in writing what is required to make the work good.
2 : Write formally to your shower man and tell him that the installation has not been installed in accordance with the requirements of BS7671, and that following a safety inspection by the housing association, the shower has been disconnected by them.
Confirm that you enclose a survey report that details the remedial work that needs to be undertaken to bring the installation into compliance with BS7671.
3 : Request that he makes good the work and brings it up to a standard that complies with BS7671 and the relevant BS7671 Certificates are issued.
4 : Advise him that he is free to make good the work himself, or appoint a competent contractor to do so on his behalf, and that the total cost of the remedial works and issue of the relevant Certificates will be borne by himself. Whichever way he wishes to proceed with the work he should supply you with evidence that those who will do the work are suitably qualified and competent to do this by means of registration with a suitable trade association, and membership of a Competent Person Scheme in respect of the electrical inspection and testing work.
5 : Tell him that for the avoidance of doubt completion of the remedial works will only occur once the relevant test and inspection certificates have been made available, and the housing association landlord is satisfied with the standard of the remedial work.
6 : Request that he replies within 14 days of receipt of this letter setting out how he intends to proceed, and that if he does not reply within that you reserve the right to instruct a Contractor to undertake the remedial work, and will expect him to reimburse you the full costs of this.
Keep a copy of this letter, and the attachments, which if you have them should include any evidence from the housing association as to the condition/disconnection of the supply, and the survey report that sets out the work required to make the installation compliant. Do NOT discuss money at this stage.
Send the letter recorded delivery, and keep the receipt. You should also state at the start of the letter that it has been sent by recorded delivery to confirm receipt.
Wait for 21 days before moving to the next stage.