BBC HD via Virgin on Demand

For reasons best known to me at the time, I recorded the HD tx of Masterchef: the Professionals on my V+HD box but decided watch it on the HD catch-up service.

Blow me, but the difference in quality was astounding! Now, I know this isn't an Attenborough-esq show with glorious vistas, but the on-demand feed had real HD 'pop', while the HD recording (I checked later) was dull, lacked detail and looked upscaled. It was pants actually.

Why is this? 

I suspect it's a compression issue (but, hey - still 'why'?), but the top tip for Virgins subs is watch BBC HD via On-Demand.