It is very easy to mark or damage the block end cap because the area that needs to be gripped is very narrow and the surface area that a spanner has access to is also very small and very soft in comparison to hardened steel spanners/grips/stilsons.  You CAN  use a standard 36mm spanner to remove the end, but as mentioned before, the surface area is so small on the flat sides that you can round the soft aluminium of the end cap. A lot of people revert to using a pair of grips or stilsons or other steel tool. You will almost certainly damage the end – steel will easly “bite” into the aluminium of the end cap and mark it.  This is where the Lawrie Spanner© comes  into it’s own.  Simply place the tube in a vice in the same way as you did for the gauge end (obviously exposing the block end to work on this time) place the Lawrie Spanner© over the end and then use a 30mm spanner to remove the end cap. Because the Lawrie Spanner© uses the six sides  (surface areas) equally to exert force on the end cap, zero damage is  expected but NOT guaranteed.  If it’s really tight, give the far end of the 30mm spanner a short, sharp whack with a rubber mallet rather than applying slow, gradual force.