Adding More Fish

 
It depends on whether you're a "small number of many species" or a "larger numbers of less species" person. I'm on the more variety side of these opposing views but there are plenty of people on this site that will disagree with me! It's personal choice.
However, we've been looking at adding replacements for losses we've had along the way and advice from LFSs suggests you need to be mindful when adding new members to an existing shoal. Sometimes the newcomers don't get accepted causing stress throughout the whole aquarium. Maybe your best option is to just add the Rainbows.

I would also get more of the fish you already have.

Dwarf rainbows are active fish and I would not put them in a 40 cm long tank. I know Seriously Fish gives a minimum tank length of 60 cm, but I've had this fish in a 107 cm long tank and they used all that length.

Do you mean boesman's rainbows? They are big fish - I know the profile on here says 75 cm tank but the profiles on here are a bit out of date. Seriously Fish says minimum tank length 120 cm.

If you ever get a big tank they would be fine - provided you have hard water 

You have me intrigued now.

The fish I know of as dwarf rainbowfish are Melanotaenia praecox, http://www.thinkfish.co.uk/fish/dwarf-rainbowfish.html and http://www.seriouslyfish.com/species/search/melanotaenia+praecox but they are pale blue. Well, females have yellowish fins but that's the only yellow on them. And they don't grow to 10 cm. These are the fish I once had.

Which species is it you like, the ones with yellow on them?

I could fit double the length on my sideboard very tempting!! I'm really enjoying keeping fish 

There's your answer - another tank/bigger tank.
Come and join me on the dark side of multiple tank syndrome. 

Yes sorry looking at the Links it's the dwarf neon, I love the colours, I think it was the right choice to add to my tetra in this tank though, there colours have really come out, I can't  believe the difference between them now and when I first got them, the red was barely there now it's a lovely bright/dark red colour
I've been thinking I could move the little tank and have two, 
This was supposed to be a Christmas present for my son but I've definitely took over,
My partner tells me off for watching the fish and not concentrating on the TV 
I

My thoughts exactly. 
You do have your family to stop you from the worst of MTS. I realised it was getting bad for me just before Christmas when I was have 2 more tanks delivered by MA, and one of the guys said "this is what my living room would look like if I lived on my own". 
Perhaps I'll update my signature/profile soon, which will show you the danger of uninhibited MTS. 
It's great to hear that your fish have settled and are now showing their colours. Sometimes it's nice to see the same fish in a shop tank, as it makes you realise how much your own fish have come on in their colours, markings, etc.