I have the 125bhp and no such issues, the 1.6 engines should be good on hill due to the very good torque profiles which is pritty much flat. They generally driver better than one would image from a 125bhp engine. I would image the same to be true of the 105bhp version

on a hill in 3rd gear with the engine at 1500-2000rpm it can struggle even when my foot is 3/4 down on the accelerator pedal.

Have you tried changing down to 2nd?

That would require mechanical sympathy and/or a concept of torque curves. Mybe the OP had a diesel previously (or a large capacity V8)