It seems the previous owner had modified the gun to work with a certain pet load or cartridge length. However, the factory .357 I intend to use (Magtech Cowboy load) is a bit shorter, allowing the rim of the next cartridge to enter the receiver and block the shell lifter.
I e-mailed Henry to ask for a quote for a replacement lifter plus shipping to the UK. Kathy e-mailed back and said just to tell her the parts I needed and they would ship them to me free of charge. To save going back and forth in case it was something else, I listed 8 parts, including the lever, that showed wear. Kathy, true to her word, mailed them to me for free.
I fitted the new parts tonight and it cycles .357 beautifully.

Henry clearly understands the importance of word of mouth and having functioning guns turning up to ranges so I just thought I'd pass it on :goodjob: I'm very pleased with the rifle and look forward to taking it down the range for a blast this weekend