When I stepped the 9.5 metre, 40Kg mast in Weaverbird, it cost me more than £200 to hire a HIAB, mainly because it was an hour's drive each way to get to Ravenglass for ten minute's work. When I come to unstep it, I'd do better to buy a 5 metre aluminium tube at about £50, for a one-leg derrick.

When Tystie first gained a 11.5 metre mizzen mast, I used the long yard from the original sloop rig as a one-leg derrick. That went OK, but it took three of us - one to hoist, two to keep control of swinging and swaying, and lift and guide the heel of the mast into place.

This is the one I use and have never had a problem with. The only difference I put an extra turn around the spar before making the tucks under the turns. https://survivial-training.wonderhowto.com/how-to/tie-halyard-hitch-knot-177169/

I have to lower unstep and step my masts on my Benford badger and have been trying to think of a cheap way.  34 feet long so CG is probably about 16 feet from base of mast.  I had thought of a bipod or tripod of scaffold but they don't seem to make it that long.

Whereas a single stayed pole, like your set up works.....as long as one has the chainplates left over for support.  But good thinking!

That’s right. For easy setting up and dismantling (and also possibly because of limited crane height), it is often easier to lift from the bottom, with just a loose restraining loop  around the mast up at the crane top. After the mast is secured in place its easy to let this loop slide back down to where it can be undone. Unless you have more courage than me and a strong second pair of hands, this loop must be above the centre of gravity of the mast – and better if above the COG with a bit of a margin.

You need to be careful doing that, though. For practical purposes, the not inconsiderable weight of halyards, lazyjack lifts etc all need to be considered as being concentrated at the top of the mast. If the mast rotates a little from the vertical, anything hanging from the top will be trying to turn the whole thing upside down, which can happen, as I know, so keep that restraining loop well above the COG just to be on the safe side.

(This has been discussed before on the forum and I believe there are proper knots you can use for tying off the lifting line near the lower part of your mast. Maybe someone will chime in again with that information - a "timber hitch' I think it might have been.)

Treat your mast-raising as a crane operation, which it is – and always be aware of other cars, people standing around etc at the launching ramp. Oh – and always (always) look up to make sure there are no overhead power lines. People have been electrocuted raising masts.

It is also worth noting that you could shorten your old mast down, for convenience, as the crane only needs to be tall enough to get the lift point (or the said restraining loop) above the COG of your mast. Hoisting from the hounds, you obviously you already know that

Here's another one: A mast leaning forward, supported by two backstays, makes a "one-legged derrick" - the simplest of all cranes. It is very easy to adjust and position the load directly over a target. 

Possibly not quite right for a trailer boat crane, but could work on a bigger boat with chainplates in the right place, as the old mast step will be aft of the new one and the crane can lean forward and be easily adjusted to get the new mast hanging directly over the target (the partners).

(Ideal for placing and driving piles in the mud as I know - got the idea from wonderful old boat designer, Brian Donovan)

You are onto it.

I had planned, with my 26’ Pelorus conversion (currently stalled), to leave the original rig in place until the time came to step the new mast. And then to use the new mast to enable me unstep the original BM mast. Anything to avoid having to visit a marina and hire a hiab.