wet vs dry suspension 1969 Wolsley Hornet

Can anyone suggest any ideas please, we have a 1969 Wolsley Hornet mk3, it failed its MOT on very minor things apart from the fact that the hydroelastic suspension will not pump up. We have replaced some of the pipework that had been damaged and the suspension pumps up and down when tried with an air compressor (suggested by the garage) however when we took it to the garage they could not get it to pump up.
What are our options? Replacement displacers seem extremely expensive to buy even second hand, would it be better to replace the suspension with dry and if so what is the best way to go about this? Do we need to source early mk1/mk2 mini subframes because the subframe has twin bolts at the front.
Any help or suggestions would be much appreciated.
:-)

I went to coil springs on my '67 Elf. The ride is great and road holding is great with no front to rear yawing. My local supplier , Heritage Garage, in Costa Mesa California http://www.heritagegarage.com had some spacers made that locate the coil springs into the rings where the displacers seat so there is no worry of the springs shifting. The springs are the ones from Mini Spares so are excellent quality and the spacers are made to a high specification out of a tough alloy. He has the three different levels of springs in soft, medium and hard. Since I drive spiritedly I went for the medium springs. I get no body roll and the suspension is forgiving enough to take long distance rides without feeling battered at the end.

The only other modification I needed to accomplish was the installation of the front shock mounts as well as have the rear radius arms drilled for a bolt to hold the rear shock bottoms.

The spring conversion for the wet subframe was $650 USD and the various other bits were another $200 USD.

This was a much better option than changing out subframes and swapping everything over.

I have cone-sprung, coil-sprung and hydrolastic Minis. In terms of road comfort, hydro is by far the best, with a small trade-off in handling. Dry cones are my least favourite. Coil conversions cost little more than cones and are available in several grades - Minitastic for example seem to be well-regarded. The downside I find is they do make some knocking noises - I think they need rubber insulators which aren't supplied.
I have P&L fast road coils on my daily car, but of course the cost is increased by the need to add shock absorbers as well.

Personally I'd try to retain the hydro. If it pumps up with an airline then it looks as though the displacers are working. I had this once with my Clubman, but perseverance with the pump and it went up eventually, after letting some of the fluid out. Perhaps replacing the pipework has created an airlock. I'm not sure if there's a technique for doing that!

Ah, I've just read the manual. "After replacing interconnecting pipes or displacers it is essential that the air is evacuated from the system and a partial vacuum created". So using an airline won't have helped... The original hydrolastic service unit had a suction device which did this, reducing the pressure to 0.9 bar. I can't easily think of a way to do this without the original service unit - I think there's a need to investigate vacuum pumps, as it's something I am likely to need myself - I have a hydro pump but not the vacuum pump. We used to have one in our science lab that would perhaps do this. Perhaps rig up a compressor to work in reverse?

The garage told you to do that? If I were you I wouldn't let them near my brakes. Putting air into a fluid system just ain't smart, and probably didn't do it any favors. You'll almost certainly need to get it on a proper Hydro unit and pull a vacuum on it. If there wasn't air in it before, there is now.