Have you heard of the Ramsey Baby Steps? www.ramseysolutions.com/dave-ramsey-7-baby-steps
Overdraft - that is part of Baby Step 0, getting current. So I would focus on paying that off. You may well have a good deal at the moment from your bank but that may not last. If you paid £100 a month to it, you would be free of that within 5 months - so several months before Christmas. You can then put that £100 a month towards a Christmas sinking fund.
I am opening up a holiday and car fund later on today, would you suggest opening up a savings for anything else?
Christmas, see above. Car fund - what would that include: cost of a car, insurance, tax, MOT, ongoing maintenance.
Birthdays - you can probably cash flow those every month but you might want to have a small pot of money for birthday presents, cards etc.
Should I continue to contribute to my emergency fund or is £6k enough?
It depends on your circumstances. Are you spending £1k a month on essentials such as rent, heat/light/water, internet/phone, travel, food, clothing? If so then £6k sounds about right.
I don't have very much in my pension (less than £100) because I didn't opt in but now I have and I've increased my contribution to the max (5%).
Great. 5% won't be the max, the most you can pay in to your pension each year and get tax relief on that, is equal to your 0.8 x gross salary, or £32,000 (£40,000 gross) whichever is lower. Minus what the employer pays in.
Putting something in pension is a great idea. If you put in 5% and your employer puts in 5% then that's fantastic. Do at least enough to get whatever your employer contributes. If you want a percentage to aim for, then 15% is a good figure... but some can go to LISA.
Do you think it's worth opening up an ISA
Yes, but is buying a home likely in the next 10 years? A Lifetime ISA is something to look at, you can put up to £4k in each financial year. It can be used towards a first home deposit, up to £450,000 value (I think, look up the figure, plus it may change in future).
If a home purchase is likely in under 5 years time, use a CASH Lifetime ISA. Otherwise a Stocks & Shares Lifetime ISA.
Try to keep focus... so if you get extra money such as from a tax refund, put it towards your primary focus area. Initially that may be repaying debt, then topping up emergency fund, then topping up a sinking fund.
Do you like YouTube, or Podcasts, or Audiobooks, or all three? Are you listening to and/or watching financial channels such as:
Meaningful Money, Money To The Masses, In Her Financial Shoes, MamaFurFur, That Mint Podcast, The Art of Money Saving, ChooseFI
Use whatever medium you like to get as much information from as many sources as possible, so you learn about how pensions and investments work.