Worcester Heatslave combi boiler - removal of dom hw thermostat?

I have a 21 year old worcester oil heatlave 15/19 combi boiler (ser No 3/88/10/22450), but the domestic hot water thermostat appears to be "lazy" (sometimes it takes up to 3 minutes for the boiler to fire up). The servicing instructions say to remove the split pin holding the phial in to the control box, but on my model, there is no split pin. (It is also situated on the top, rather than the front as the diagram indicates. It appears to be a push fit, but it is very tight and I am reluctant to apply too much force. Can you confirm that it is likely to be the the thermostat, that it is a push fit and what would be the best way to remove it?