Bathroom extract fan problem

This is another problem of the "where to put the isolator switch" fan!
Over the week-end we did the first part of the flat bathroom refurbishment. We've put up the false ceiling and the new lighting without too much problem.
However the extract fan is going to be difficult. The original fan doesn't work so needs replacing, on removing the old fan I found that it has a completely inaccessible junction box mounted a couple of feet down the service duct which is brick with no access holes near where the junction box is! How they got it there I'll never know. OK, so I can simply ignore the old wiring (I have found the other end of it so have disconnected that) and put new wiring in.
The real problem is that the old extract fan uses what looks like a 2" or so diameter tube up the service duct for the extract. Are there any currently available fans which can produce a reasonably effective extract up such a small diameter pipe? If so where am I likely to be able to get one? I would guess that the tube must be quite long as it (hopefully) runs up through two storeys to the roof.
Alternatively can I just extract into the service duct? Is this allowed in general or would it depend on the particular block of flats? I think there is only one other 'internal' bathroom in the block.
I *might* be able to extract into the old gas flue (the new gas boiler has been moved and has fan assisted flues which no longer use the service duct). I'd prefer not to do this though as it would be lots of hassle and I don't even know if the old flue is still in place and/or hasn't been blocked up.
Chris Green (

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