Over the years I have covered a lot of different Mk 6 helmet covers on the blog, DPM, DDPM, UN Blue etc. The final iteration of the cover, however, was in MTP camouflage to go with the new pattern of uniform introduced in 2010. This cover would see service in Afghanistan until the introduction of the Mk 7 helmet and would remain in service with other units that were still using the older pattern helmet for years to come:

The design of the helmet cover should be very familiar now, consisting of a shaped bag in MTP, with a drawstring fastener at the mouth to secure comfortably around the rim of the helmet. This is pulled tight, tied and the excess tucked neatly inside:

The top of the cover has a web of elastic sewn to it that allows camouflage to be secured to the helmet to further break up the outline of the wearer:

These also provide a useful place to write your name so that your helmet is easily identifiable amongst a large pile of them, which is what I did with mine when I used one.
Where the helmet cover passes over the chin strap mounting position, the fabric is reinforced as this is a point of higher wear:

Finally, a standard white stores label is sewn inside the helmet:

Several sizes of cover were issued to match the different sizes of helmet available, although smaller sizes could be made to fit with difficulty (and much swearing). With the replacement of the Mk 6 helmet and the relatively recent use of these covers they are easily available on the collectors’ market for just a few pounds.
TOP TIP:
The official guidance on fitting is usually lost before it reaches the storeman/woman. When fitting a helmet cover, wet it and wring it out first. Makes it MUCH easier to fit. Try it.
(I can’t speak for the Mk7 or Virtus helmets – well after my time!)