Planning to return to England with my family, for the first time in 8 years for my mums 80th birthday - Mid April this year. Could anyone please give me any advice on the cheapest return flights (preferably Brisbane to Manchester) and how to get them. Just had a look on the internet and cannot believe the cost for 4 adults. Over 15 grand is simply too expensive.

It'll take a bit of googling, but you won't need to pay over 15 grand.

Before you start to search, have an idea of your tolerance level for the number and length of time of transit stops.
For example, Webjet have a combo Emirates and Qantas fare, Brisbane - Manchester return for 4 adults, for a total of $10,126. Depart Brisbane 12 April, depart Manchester 11 May.
2 transit stops each way, Singapore and Dubai. The outbound transit times are 4 hours 20 minutes (Singapore) and 2 hours 10 minutes (Dubai), which are good. Coming back's not so good, with nearly 14 hours in Dubai and 12.5 hours in Singapore. Some people can tolerate that, I'm not one of them unfortunately!

Flight search engines I've used are:
Webjet
Google Flights
Kayak (make sure it's the Australian site to get the price in AUD)
Expedia
Skyscanner
Tripadvisor
There are many more, some ok, some as dodgy as hell so take care.

If you find a good fare with stopovers and transit times that you're happy with, check the airline's website to see if they offer a similar price. I prefer to book with the airlines themselves if possible, but will book with some reputable online travel agents if the fare is significantly lower.

If you want to PM me with your dates, how many stopovers you'd be ok with and how much time you can tolerate in transit, I don't mind having a look for you. Will have to be this evening though, got a fair bit on today.

Hi, I have just looked at some websites here in the UK and I can find $11’057 for four flying out from Brisbane with Virgin on 13//04/23 (2 stops) and return to Brisbane from Manchester Qatar (1stop), hope this might help, good luck.

I had a quick look on Kayak and I chose the Queensland school holiday dates and it was 12k for 4 adults. Worst case scenario over school holidays.. If you can do outside school holidays it might be a cheaper. Check going via US too.

Easter is April 9th, that will have an effect on the prices - would also probably match with the Qld school holidays. Travelling after Easter and getting there only a few days before the birthday might bring the price down a little.

If booking from Australia is expensive then try to separate the booking into two: 1) flights to Singapore on Scoot 2) the rest of the journey. Granted you might need a stop-over or at least enough time to collect your luggage and check it in again, but I've booked my last few flights this way as including Australia in a long itinerary seems to inflate prices to ridiculous levels.

Only carrier that was semi-reasonable during some searches (starting in Europe though) that would take me all the way to OZ was Finnair, even though the last leg was a choice between BA and Qantas.

Be sure to leave enough time between flights to cope with any reasonable delay in the first flight, because the airline won't be liable if their delayed flight causes you to miss your next flight.

That is correct. As they are two separate bookings, the second airline does not have to accommodate any issues that you encounter with your first flight. If you miss your second flight, you might end up having to pay for another ticket for your onward journey.