\why is there warm water in the base tray of my Myson ?

I assume it is a condensing boiler. If so, it could be that the outlet pipe draining the condensate away has frozen. This is a common problem in the current weather. You may be getting a build-up of this water leaking out of the container for it which is located within the boiler casing. On my boiler (which is a Vaillant) the condensate pipe coming out of the boiler is a push fit. With a container held underneath the boiler, simply pull off this pipe. The water which has built up should flow out. This will allow the boiler to re-fire. I've cut the condensate pipe a few inches under the boiler and am collecting the water in a bucket. It comes in short bursts because of a siphonic action out of the container in the boiler, and should be a manageable amount which can be poured away each day. When the thaw comes, I will replace the output pipe aiming to get it better insulated particularly where it goes through the wall and outside. I hope this helps.

BH
January 2010