So I filled about 1.2-1.3l of oil in the box through radiator pipe but no improvement.
If anything it's worse now, could be fault developing further or the new oil has introduced more lubricity so it's slipping easier. It's slipping in 4th as well now and it's less often that I can get all gears to work OK and when they work kind of OK it's only for a short time before they start slipping again, I think it was probably slipping in 4th before as well I just did not feel but now it's more pronounced. 5th still working fine. After some more research I found out that the default failsafe mode for the box is 4th but if there is a problem with forth it goes to 5th. And if it goes to 5th in failsafe it's usually issue with A clutch, either broken drum or no oil pressure.
Interesting fact on Audi oil container it says that if stored in sealed container it can be used for 3 years since date of manufacture, but when filled in box it's good for life
Sent e-mail to Planet Gears in Peterborough this morning an the chap called me back said he was sorry about delay in calling me back as he was travelling to their London branch today, going from the phone number he was calling from
https://reengineeredtransmissions.co.uk/ He was not going to speculate what was wrong and how much it will cost to repair but as an example he gave me some prices.
1.Remove box, half rebuild, refit, new filter and oil about £900-950 +VAT.
2.With a full rebuild less valve body around £1400+VAT
3.If there is something wrong with valve body it's £150-350, he did not specify and I did not ask but I guess +VAT
I asked what's the difference between half and full rebuilt, he said half rebuild includes all soft parts, full rebuild includes all wear parts.
I did forgot to ask about torque converter but I guess that's not included.
Not sure about next step but I guess changing oil and filter will probably be waste of time and money although oil can be had pretty cheaply
https://www.ebay.de/itm/20-Liter-Aut...item54742d0d55