You could try to either store the whistle in a fridge for 30 minutes or hold a hairdryer on the slide for some minutes. This would cause the metal to expand or shrink and possibly make the slide moving again.
A combination of the two might work. Freeze the whistle to contract the metal. Then, heat only the outer female half of the slide to expand it. If you have access to a small blowtorch (careful!) that will work better than a hair dryer.
The best thing would be to send it to Colin for repairs but international shipping can take a long time and cost a lot. Another option is a local woodwind repair shop.
Of course, if the whistle is in tune and lined up properly you could simply forget about it.
Why Colin? It’s a Kerry whistle. I emailed Phil Hardy and he’s willing to help, but not only is shipping expensive, there is also a postal strike in the UK.