I registered my own company, I do all my own payroll (even if you don't pay yourself you need to do the HMRC RTI stuff, I just click 'non pay periods' every few months), my VAT (flat rate scheme, piece of [VAT-free] cake) and keep detailed records. I have a spreadsheet that tells me how much corp tax I'll need to pay, I deduct any business expenses, then pay myself the rest as dividends.
I send the records to the accountant at the end of the company tax year and she charges me £250 to do a short statement for Companies House plus my own Self-Assessment.
I use HSBC bank for my business account (yes, you *must* have one as your payment from clients must go to an account in the same name as the contracting party and the company that issues the invoice).
If you are a Ltd co there are also a lot of rules around what you put on your invoices and headed paper etc. For example, you must put your Co Reg number and your VAT Reg No on all invoices, and the co Reg no on all correspondence - even emails (so, design yourself a footer and use it). It should also be on your website and business cards if you use them. And, in theory, your registered offices but I've never bothered to be honest, since it's my house. I do have the 'company file' in my office, with all the reg documents and the share information. It is a different colour to all my other folders and clearly marked.
Oh, and every time you pay yourself a dividend you must do a 'company resolution'. I do this, print it, sign it, stick it in the company file and keep a soft copy on the PC.
I would say though, if you have only one 'customer' and they have given you an 18m contract why isn't that just employment? It would be very hard to justify that as a service contract. Usually I tell companies to only give 6m contracts and then 'extend' them. Also, do you plan your own holidays, control your own working days, can you work wherever you like, do you provide your own equipment including using your own/your company email address? If the answer to those is 'no' then you are at very high risk of being found to be employed and the 'contractor' status a sham if HMRC does an inspection.
For me, I have an employed job 3 days a week and three separate clients. For the clients I do most of my work from home, if I go to their site I take my own laptop, I don't use their email addresses, I do most of the work ad hoc (i.e. no set days, no set duration except with one they give me 6 day call-off contracts at a time, I may or may not use the 6 days), I tell them which days I am available (next week I am having new carpets fitted, I don't need to tell my client this, I just say "my next day I'm coming in is Friday").