I have looked into nannies but I'm put off by the tax and the like
There are several nanny payroll companies who can take the hassle of doing payroll off your hands. Typical cost is around £200 per year, which may sound a lot but at under £20 a month it saves you having to spend time learning how to do payroll/PAYE and gives you access to advice line and legal support.
If having a nanny is something you want to consider, then best to post on the Childcare board, rather than in AIBU.
It's hard as I don't think i really want them to bond closely with one person.
You want your children to be happy and whatever childcare you choose they will be assigned a key worker. They will form a bond with that keyworker (and other staff if a nursery). In my experience (over 20 years working with children) every child knows the difference between Mum, Dad and Childminder/Nanny/Nursery Nurse. So please don't worry about them forming a bond with someone.
I do need complete reliability
Nothing is going to be completely reliable. Nurseries close, people are ill, travel difficulties, heating fails, all sorts of things happen. As a nanny I have not had time off for illness, in over 10 years. That does not mean I am 100% reliable, I could be ill tomorrow. With a nursery you do have more people around but you also have more children around, more viruses spreading around, so initially children often become ill. So whilst the nursery may be open, your children may not attend.
A benefit of childcare in your own home is that the nanny can care for mildly ill children, so a child with the sniffles or an eye or ear infection can still be cared for enabling you to go to work. Sure the child concerned may want you to stay at home but you could decide if that was really reasonable given their condition.
being harsh/draconian with the DDs, ignoring them - that sort of stuff.
Can happen in any childcare setting. I have seen adults being harsh with children in nursery. It may come down to experience of the workers, lack of close management, or just people's personalities. At least if using a childminder or nanny, you get to choose that person - you don't get to choose staff at a nursery.
thinking about it that it will have to be a nursery as I've absolutely no back up in the event of illness or holiday so nursery it is! 20 month old and 5 month old - would they be allowed to be together?
Depends on the nursery. Certainly over time they may split up and then come back together again.
Time - you are currently looking at what is best for Now, not what will still work in 5 years time. Is that something you should be considering? A childminder or nanny can care for your children for many years. I am currently in my 7th year with the family I nanny.
School may seem a long way off but it is probably only 3 years away, so when that happens your childcare needs change. A nursery would probably then not work unless it was very near the school and had a before/after school club.
Lots to look at and lots of pros and cons to each type of childcare. Only you can decide what is right for your family. It sounds like you are likely to be looking at Nursery initially, so go and have a look at a couple of local nurseries, see if they offer what you need now and find out what they could offer as your DDs get older.