Johnson & Starley J15-22 mk3 Modairflow Fan problem?

Hi
I have the above warm air system in a 2 bedroom house. I came home last week and the house had a very musty burnt smell. I found that the fan would not turn and was very hot so i brought a new fan thinking the fan motor had burnt out, as the motor really smelled of burning.
The same problem is still happening, after 20-40 minutes with the heating on the fan just stops and will not start again and the fan assembly feels really hot and does not seem to cool down and feels like it is getting hotter. There is no problem with the pilot light as this is still alight.
I was thinking, if there was a blockage and have been to the loft and felt the flume/pipework, the pipework is slightly warm but the bit that goes through the roof is very hot. I took the pipe off and cannot feel any wind/breeze from the bit going through the roof, is this normal.
I have now turned the gas off to the unit and I will be getting an engineer out but any advise would be appreicated and if anyone knows of a J&S specialist gas engineer.
Sorry to write a long winded chapter.