Hi go2khurram - it's my appeal Mandy has just linked to, I think, as we had an OOC appeal for AGSB last summer.
The Sept 2020 cohort is a newly large group (PAN of 220 ish where previous years were 180 ish) and it sounds as though competition was fierce yet again. In 2019 they admitted only 9 OOC children in the main round and the lowest qualifying score was 390 compared to the in-catchment qualifying score of 334. OOC children are ranked by their score - you can ring AGSB for the lowest qualifying OOC score and compare that to yours, but it will probably be sent through in the appeals paperwork.
I would recommend that you read through Etienne's excellent FAQ on appeals -
https://www.elevenplusexams.co.uk/appeals/, mostly in category C which deals with oversubscription.
It is not usually recommended to use a professional to lead your appeal. We found the actual hearing as informal as it could have been, with the panel and particularly the panel chair helping us through to make sure we were fully heard and fully understood the process.
Bluntly, they don't care that your son would rather have got AGSB than Urmston. They don't care that it would be easier for you as parents for a few years. You need to work out how you show that AGSB is absolutely the best thing for your son, and what AGSB can offer that Urmston can't (any particular subjects that aren't offered at Urmston, or clubs that only AGSB offer?).