So have used the big beast once more with the washers on the spindle as spacers for that extra 5mm or so. Does give a good cut height comparable with utility mower - getting better with knowing how to get the best from it e.g big long straight runs, don't mess around with areas you can't get just bomb on and come back later with the utility mower or trimmer.

The skirt still digs in too much in some spots so ordered some stepped blades as long discussed,


Ok well based on the terrific help of Norm & Max i hunted for some suitable blades with a decent step in them. Didn't want to spend a heap as wasn't sure this will be a long term solution. Ended up grabbing these:
https://www.ebay.com.au/itm/234914543776

Asked the seller in advance if they were 3mm & heat treated. They took a full week to reply simply writing '3'. I assumed they're blades they'd be heat treated, so sure enough a bunch of mild steel painted ones turn up. Go figure - I've emailed asking wh they couldn't have just answered the query instead of deliberately replying selectively but not end of the world & was easier to mod than hardened would have been.

Used round file to enlarge the hole from 13mm to 16mm. Not overly hard. Then cut down the blades to the functional length of the flat ones, did the first one with an angle grinder - that was a tad messy - used hacksaw for the next. Much better. Cleaned up on bench grinder and put nice thick angle on them.

In hindsight I do wonder if as suggested drilling a new hole in the disc carrier further towards the centre would have been easier. I would love to know if the experts think that a 13mm round hole would be suitable as I would not be able to easily replicate the normal 'half round-half flat' holes that the bolts are meant for. Unless there's a trick to do I'm missing e.g drill 10mm hold and expand with small round & flat files?

The disc carrier is 3mm thick itself - so plenty thick enough, but unsure best way to do extra holes IF that was deemed feasible ? Could be handy option vs modding another 6 blades & I now have 8 x 13mm bolts etc.