Air Conditioning Top Up

[Lancia 2000]

Hello all
I bought a kit to top up the air con on my 2000 3.7. I've been searching under the bonnet for the 'low pressure' cap. I can't find it, I'm not even sure I've found the aluminium pipes!
I've just looked through your service manual and it indicates the system shouldn't be topped up anyway. Is this correct? Main dealer empty and refill only? My dealer in Chandlers Ford is very expensive and I got done the last time I went.
Any advice please?

Gary