From some searching, I think paying an electrician to install a 32A CEE socket (often called "Commando" in Britain after a particular manufacturer) and then installing the EVSE yourself would be similar. Although there do seem to be some safety requirements for the socket if it's to be used for charging EVs, which increase the cost.
I talked to the electrician about using a CEE16 connector to charge the car. He said that this should only be done if you don't load it near its max (16A) for long periods of time. He's seen multiple CEE16 connectors that deformed condsiderably after a year of use with regular charging sessions at 16A. If you load it to 10-12A, it should be fine.
I have servers and UPSs and so on connected with these sockets, drawing a high current 24/7. Just like every datacentre in Europe, UK included. They are only very rarely unplugged though.
Is it a case of using a high quality socket which costs 2-3 times as much?
Not the person you're asking, but it might also be where it is installed. The connector I looked up online is rated to +40C, and being located in direct sunlight on a hot day could derate the connector compared to a temperature controlled environment. Add in some dirt or corrosion in the connector and it will generate more heat. Like you said about buying a high quality connector, something with more margin in terms of current will help prevent problems from developing.
The electrician may have been referring to the 16 amp connector specifically being too close to the charging current.
Also, the £900 includes 20% tax.