You can try RMC's suggestion but doubt it will be enough.
Having just reread your initial post, salary attributable to UK duties under your employment will only fall outside any UK income tax charge if the contract of employment was with an Australian employer and you worked in the UK for the tax year for less than 183 days. If contract with UK employer then UK tax payable.
Maths, when you say "Don't include non-UK source income" are you saying that the non-UK source income should be listed in HS302 but the value should not be included in the pay figure at Box 1 of the Employment pages?
Assuming employer re UK duties was Australian (hence DTA may apply) include in HS302 only salary attributable to UK duties plus UK tax paid thereon.
Complete Residence etc Pages to the Tax Return. I would then be inclined not to fill out Employment Pages as there is nothing to enter as in HS302 have excluded UK salary any therefore nothing left to disclose. Make note in white space in Tax Return to this effect.