cgt on uk property for a non-uk taxpayer

I am UK citizen who moved to Australia 35 years ago, and am now an Australian citizen and Austraian for tax purposes. I have a "NT" code for income tax in UK so things like company and state pension is not taxed in UK, but taxed in Australia. About 30 years ago I inherited a small portion of my old family home in UK along with my siblings in UK, when dad died. Since then mum has lived in the house. I have not received any income from the house, although I have paid some maintainace on it. Mum died recenly and property has just been sold. As a non-resident of UK for tax purposes (and not knowing a thing about the present UK tax system), can someone please tell me my liabibility for UK CGT I may have? Please include things like tax rates, allowances and calculations. Property value when dad died approx 90k pounds, sold for 430k pounds (yet to work out things like selling costs), my share of property 1/8th. I have been told I need to pay up within 60 days. How do I do this, especially as the only UK tax Identification I can find I have is a Government Gateway user ID. As I live in Australia and things sent by post takes about 3 weeks each way and phone calls cost a fortune, I would need to do this online if possible. Thanks, Brain.

Hi,
You can see guidance here:
Report and pay your Capital Gains Tax
Thank you.