Badvoc - I am the same with Xmas, car tax, birthdays etc and I am trying to start budgeting for them monthly in YNAB. Problem is that the"big one off's" still come around while you are trying to build these categories and so I find you steal from "Xmas" to pay "car tax", etc.
All I can say is that every little helps. I knew I had a big birthday month in March (plus Mother's Day) and tried to put a little aside in Feb, but ended up pulling most of it for another expense. I managed to leave £10 there though which was £10 less to find this month. I have a break in family birthdays next month but I will put money aside regardless so I can build up for June (4 birthdays and Father's Day) then October (DS1's 16th + 2 birthdays).
Car tax I realised is in 4 months, so HAD to start putting money aside this month. I have managed that. I've also started off Xmas fund with £50 but may steal from that depending on how much DS2's birthday ends up costing next weekend. The way I look at it is that even if I only leave £10 in the Xmas fund, it is £10 more than I had saved for it last year!
When is your road tax due? If after October, it can be done on monthly Direct debit from that point so perhaps doesn't need saving for?
I would work out your costs for birthdays/Xmas/car stuff and divide by 12. That's what you need to save. If it's too much, where can you trim it down? I assumed £20 average on most birthdays but when I added it up and included a bit more for DS birthdays, I was shocked! So I reduced to £15 average but am going to go nearer the £10 mark where I can.
For Xmas, we now do a family secret Santa with a £30 limit for adults - there are lots of us and it saves a fortune. I'm hosting this year so it will inevitably cost me more than usual, but saving on presents via secret Santa will help no end.
I was very depressed too when I started YNAB and found my "big one off" pots just kept getting raided, but I DO have some money in them now, albeit only a little, and I think it will get easier as time goes on.