Fixed! I ordered
http://bersa-tools.ru/goods/Remkompl...1127548196-BMW, a replacement membrane and cap. The membrane can be purchased standalone from bersa-tools.ru, or from
https://vanos-bmw.com/product/111275...53-11127548196. vanos-bmw.com also sell the aluminium cap
https://vanos-bmw.com/product/cup-fo...81-11127548196.
Plenty of sellers sell a complete plastic cap / membrane for the N53, it does not fit the N53 even though it is listed as such. The N53 requires a full width membrane and a flat cap on the underside, the plastic caps have a screw type fitting which does not fit into the inner area of the CCV breather valve on the N53. The membranes are sealed into the plastic cap which means they cannot be transferred to the original cap, they are no good for the N53. It was obvious as soon as I removed the old cap but prior to removal (and I didn't want to remove the cap until I had the new one ready) it was guesswork. The support from vanos-bmw.com was excellent, I bought the original plastic cap from them and they refunded me after realising it was not N53 compatible (and have revised their listing) and pointed me in the right direction. The parts above are what you need, either seller will sort you out and I've bought parts from both.
You will need the membrane and your existing / a replacement aluminium cap. The advantage of the aluminium cap is that it becomes reusable if the membrane fails again and it should prolong membrane lifespan by dissipating heat more effectively. Personally I think the old cap could be salvaged and you'd get away with the membrane replacement in isolation, although the aluminium cap is more expensive I'm glad I opted for it. Reusing the old cap would require filing away the raised bits on the inside and good fortune when you're trying to remove it from the cover. It's a gamble that might leave you without a working cap. Treat yourself and buy the aluminium cap!
Here are a few photos of the repair, it was done in two parts as I ordered a plastic N53 cap which did not fit but served as a temporary repair until the flat aluminium N53 cap arrived today. The poor fit of the plastic cap (after I sawed off the extra material!) meant there was an air leak and a whine. Seemingly only available from Russia both took 9 days to arrive after ordering, door to door. The repair is easy enough, the original cap needed warming with a hairdryer to soften the glue and then hammer to tap a screwdriver head into it. Encouragingly the old membrane was completely torn and definitely in need of replacement. Although the old cap came off cleanly the retaining clips on the valve cover broke off. To ensure the new cap has a good seal without the retaining clips I've sealed the cap using liquid gasket (silicone based) which will play nicely with the membrane material and remain flexible should I need to remove / replace the membrane in the future. With the aluminium cap and new membrane there's no whine from the engine and the oil filler cap vacuum is reduced.
FYI my membrane started to tear around 140,000 miles leading to the whine / whistle noise and failed completely a few weeks ago (155,000 miles) which meant I had surging power and a terrible idle. If your car is already whining chances are you've got ~10,000 miles left before the membrane splits completely.
I do not know why BMW refuse to sell the membrane separately. I was quoted £550 for a valve cover + fitting! Changing the valve cover in full requires expensive parts and a lot more effort to fit. I suspect as these cars get older the Russian parts will become easier to get hold of on eBay, the best I could find was the useless plastic cap variant from China and the USA. Only the Russian websites sell the aluminium caps and standalone membranes. Even with having to fit this twice I'm still less than two hours in, I'm a dab hand at removing the engine cover and other bits due to N53 injector fun but there is nothing difficult about this job. I finished this off today in storm Doris and still had no issues.
I hope this helps other N53 owners. I'll post a long term update on oil consumption as I've been using oil (no visible smoke, no leaks) and this is a known cause of it.
Original cap removed, tear in membrane visible.
Cap removed and the revised N53 innards on show, this is why the plastic caps do not fit.
Tear in the original membrane visible.
Original cap on the left, incorrect plastic cap in the middle (after I sawed off the extra material!), and the aluminium cap on the right. Notice the difference between the height of the incorrect plastic cap and aluminium caps, particularly the level for where the cap would sit.
New cap installed.