Fair enough. As I said before, the "too small a job" issue may be a bit of a problem for you - it may cause many electricians to not really want the job and/or (not unreasonably) having to charge much more than you would like in order to make it worth your while. It's a pity that he could not do it whilst he was there (** but see below).
It's very difficult with electrical work. Whilst what you say is sort-of-true, as has been said, the problem with all these 'recommendation websites' etc. is that they are so easily abused/manipulated that they can very easily mislead. With work that can be, at least to some extent, judged by the general public (decorating, gardening, some building etc.) one can ask to see work that they have previously done - but, as I said before, other than for superficial aspects, it's generally not possible for an 'ordinary person' to judge the quality/safety of electrical work.
The job should be so simple and straightforward that you probably don't (shouldn't!) need to worry too much about who does it - i.e. only the most incompetent of electricians would be able to do much wrong when replacing an RCD.
** However, it has occurred to me that there is an aspect of this which we haven't considered, and which might make the job a little more 'complicated' ...
... is this RCD in your Consumer Unit? If not (i.e. if it is a 'standalone' device), is it connected directly to your electricity meter? If it is, then it would be necessary to get your electricity supplier (actually 'Distribution Network Operator', "DNO" to remove their service fuse before the work could be done and then replace the fuse after it had been done. If the RCD is not in your CU, then a photo of it and its surroundings (i.e. what it's connected to) would be helpful for us to see.
Kind Regards, John
