Thanks for all the ideas.
Its looks like 90% of the problem has been solved by changing the molecular
sieve GC column for a new one. I still have some tailing but much
improved. I had baked out the old column in two overnight sessions at 250
C, it looks like this is not enough, at least not to recover the old
column. The GC oven I use on this system can only just reach 250 C, it may
be that I will need to use a different oven that will reach at least 300 C
to properly bake out these columns. It may also be that the slight
residual tailing I still see is because this new column may also need
further bakeout before it is fully functional.