Were very common in the period 1941-1944, depending on the unit. Decal transfers were used by many units; the Seaforth Highlanders had a cap badge on a tartan square, the Calgary Highlanders had a 3x3 set of red and white checks emulating the dicing on the glengarry (and these are commonly seen in wartime photos of training in England), and if you look at Ken Bell's books you will see other examples, even on the Continent. The Chaudieres had a maroon/white/red flash divided diagonally, and you can see these in pictures taken on Juno Beach. The Maisonneuves had a white fleur-de-lys on their helmets; Bell shows a couple of
them at a funeral service in France or Holland somewhere. I can think of specific examples, backed up by photographic evidence or talks with museum curators, for the Royal Montreal Regiment, Regina Rifles, Royal Winnipeg Rifles, Hasty Pees, and 48th Highlanders of Canada.
In action, they were covered by the helmet net/scrim and probably painted over or otherwise deleted in some cases, or never actually added as the great influx of new replacements joined the units after the first month of action. The number of men serving in infantry units in late 1944 that had actually been in the UK with those units in 1941-43 was rather low, especially in the frontline infantry companies.
A replacement from a light anti-aircraft regiment in October 1944 probably wasn't given instructions to mark his helmet or to receive a new helmet with markings on it. I suppose that was up to individual units - though bear in mind the first thing replacements did receive was shoulder titles and regimental head dress. See Frank Holm's book about joining the Calgary Highlanders as a replacement signaller after Normandy. I also have a set of QOR dress instructions that does stress highly the desire to have men dressed "regimentally" - though the orders do not mention helmet markings and the regimental museum denies that they adopted them at any time (though photographic evidence suggests something else).