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We carry a Brompton bike under the fixed seat, so are tight for space. The handle of the coach bolt that anchors the seat prevents solid objects (such as levellers) from lying flat. This wastes valuable space.
Replacing the handle with a standard bolt gains space, and we had a flat welded steel plate made to replace the hollow tube installed by Murvi.
Levellers pushed up 8cm by bolt 
Anchoring bolt with protruding handle
Plain bolt compared to original 
Original anchor with new bolt
Levellers lie flat; 7cm gained 
Flat welded steel anchor
The bolt I used was M12 x 30mm from ebay
M12 FLANGED HEXAGON BOLTS HIGH TENSILE 8.8 PLATED STEEL FLANGE HEX HEAD SCREWS
Most of the gain was from replacing the bolt. I had the new anchor bar made by a local steel fence manufacturer
Nick Mawby
We also did this, and it makes a lot of difference to the use of the space. As an alternative to ebay, if you live in the right sort of locality, then Agricultural Engineers are a useful source of bolts etc. Most times, a handful of bolts cost ‘lets call it a pound’