OK. I had success with removing hoses with the aid of the hair dryer. It really worked quite well.
I was able to narrowed the problem down now.
I was able to connect a hose directly from the strainer to the pump, bypassing the rest of the hose. Impeller spewed out water nicely.
I was also able to put tubes down the hoses from the strainer to the transmission - all clear.
Also checked the hose from the transmission to the pump - all clear.
So the blockage is definitely where the water goes through the transmission.
I narrowed the blockage to a space of 6 inches. The pipe runs at the back of the transmission near the shaft coupling low down. I've tried backflushing and forwardflushing under pressure - nothing at all. It feels like a total jam.
I might have to remove this from the transmission if it is possible.
Makes life easier.
Has the boat been out of the water, or did this just start when an impeller failed?
The boat has been out of the water for a short time for an antifoul, and stern gland packing. The shaft had to be uncoupled to get to the packing as there was little room from the half coupling to the gland.
The impeller has now proven to be fine (with the old and new one)
I wonder if there is some type of valve in the this transmission cooling, or a barnacle has somehow got in here (unlikely) or part deliinated hose as suggested.
I'll have a chat to the local volvo mechanic to see if it is easy and uncomplicated to remove this transmission cooling pipes
Thanks for all the help. It has been good to talk it through
Any other suggestions?