I have bought a Citigo SE, registered September 2017. The first service recorded was aan oil change service in july 2018, then it had a multi point check and a minor/interim service in April 2019.

When is the first major service due?, I'd appreciate any advice

40,000 miles/ 4 years i'd expect.

The oil service carried out in April was done very close to the current mileage of 17000. I read somewhere that the first major service is due at 20000 miles or 2 years

Inspection service would be 20k with extras like plugs @ 40k

Oil change service every 15000km or 1 year. Plus inspection service every 30000km or 2 years.