Hi Susan,
Older installations may have the RCD prior to the consumer unit, very old may not.
My advise to you is to use a regular electrician who would call you if there are any failures whilst he or she is doing the inspection, then if it was going to fail you could authorise the changes and achieve a pass.
The RCD is there to protect a number of things, one being the cables which are aging too. I would also say if you have an electric shower you should have an RCBO protecting that.
So bite the bullet, even update the panel as you will get an RCD, latest box and protection for between £300-£500 depending on the electrician, whether they are VAT registered and the size of your property. You will automatically get a certificate so you should save on paying for that separately.
It’s not worth taking the risk
Be safe - be a good landlord