Hi there, hope I can clear these up.
If I currently achieve 29mb down and 8 up on the Plusnet Unlimited package which is advertised at "up to" 76mb will these be cut in half if I opt for a package that is advertised at "up to" 38mb? or will my line deliver the same either way.
Your line will deliver the same either way.
If its the latter then presumably those services are artificially constrained to make it possible for the firms to charge more for higher speeds.
That's not the case, the issue is that fibre (like ADSL) is dependent on line quality and more importantly distance from the cabinet. That's why people are seeing variable speeds. If you have a fibre estimate of 65Mb say and order the up to 40Mb service, then you should see the full 40Mb. If you upgrade to the up to 80Mb service though you'll not see any speeds faster than 65Mb.
Likewise if your line estimate is below 40Mb, you can upgrade to the 80/20 variation (and may well see an increase in upstream performance) but if your line can't handle speeds faster than 40Mb downstream then upgrading will do nothing to change that I'm afraid.
Hope that helps explain.