I believe they're 16mm.

 

https://www.halfords.com/tools/hand-tools/all-hand-tools/laser-spark-plug-spanner-t-bar-16mm-686737.html?istCompanyId=b8708c57-7a02-4cf6-b2c0-dc36b54a327e&istFeedId=367c5610-f937-4c81-8609-f84582324cd6&istItemId=xmlxxtmal&istBid=t&_$ja=tsid:|cid:11902546034|agid:113456751337|tid:pla-331014970735|crid:487900199209|nw:u|rnd:10046505597697047896|dvc:m|adp:|mt:|loc:1007345&gclid=CjwKCAjwoP6LBhBlEiwAvCcthBzv2TcHYDsD1ciUqhFnWqzTWT0skHneSDHRK3-9Mp51tZsEIJQdQhoCPMkQAvD_BwE

It’s made for the job but I used to prefer a plug socket with appropriate extension bar and ratchet (1/2” drive) to get the leverage to remove the plug. 

1 hour ago, Aj77 said:

I believe they're 16mm.

I prefer a suitable plug box for my half-inch socket set.

Yes they're 16mm. I changed mine a few weeks back.

 

I used a deep 16mm multi spline socket with extension bar onto my torque wrench.

 

Got them straight out without any problems.