I am looking to do a loft conversion in my house, but the roof is
supported with trussed rafters. I am looking for some advice on how to
create an alternative support for the roof so i can take take these
out and create some usable space.

The cheapest method is probably to remove the entire roof and build it again
properly. Trussed systems are simply unsuited to conversion. They are
computer designed to save every cm of wood and only resist the loads used in
design, which did not include several people and furniture.

Modification of the existing would be fiddly, and, in all probability, more
expensive because of the labour cost.

An additional advantage of entirely reroofing is that the roof covering gets
relaid and you get to install breathable membrane felt which lets you
install more effective insulation.

Christian.