Thanks to everyone who came to the talk on an introduction to cross country flying. Both the newer pilots who came along to learn and experienced ones who came to support and contribute. Much appreciated!

One thing I wish I had given better advice on was on altimeters. Nigel Page very kindly got in touch and I'll quote him directly:

... you did say that in flight changes in air pressure was a negative factor in barometric altimeters.  This can be a problem if a heat low develops but is seldom so in the UK.  Also in PG and HG we do not fly in conditions where isobars are close together because it is too windy.  If it is at all windy we end up flying along the direction of the isobars so there is little pressure change.

Personally I don't want to be reliant on GPS reception for my altimetry.  I have flown with both GPS and Baro altitude displayed on my competino with the GPS altitude wildly out.  However I don't put my flights into the leagues so I don't have the concern of scrutiny of my traces, but if a pilot sets and uses his baro altimeter properly he should not get his GPS trace rejected. 

So in summary: Barometric altitude is more reliable and the variations during a days flying are small compared to the fluctuations of GPS. So long as you set your altitude correctly for the height of launch above sea level before you take off then barometric is the thing to go for. Most instruments are set up to work this way. In terms of staying safe and legal and out of air space, Barometric altitude is what we should use.
Cheers.
Pat