They seem to be fitting some reconditioned 3 phase meters when removed from previous premises....................try asking the question through complaints handling?

was it ever fitted?

SP has recently changed my single phase meter that failed - to a new second-hand one - but we are considering going three-phase..

When I called yesterday they said I did not need a three-phase meter..  

@AOW   To fit a 3 phase meter you need first a3 phase supply which is the responsibility of your Distribution Netwok Operator, in some parts of Britain this can be SP Energy Networks which is a different company to SP retail. .    3 phase supplies are common on farms and factories with high powered electric machinery.   All domestic properties have single phase supplies.    Why are you thinking of getting a 3 phase supply?  It is not cheaper than single phase and it is only needed for very high consumption. 

@AOW   This is a useful description

https://topcharger.co.uk/can-i-get-three-phase-power-at-home/