Haven't been actively pursuing other leads yet because of the fact that I have a three month notice period and people don't seem to be willing to wait that long.
Just go on any of the big job sites, search "Junior Ruby on Rails" in your area and take a look at whats available. If you find some good jobs, apply, go for an interview - if you get an offer put that to your current employer, maybe they will match it, if not you now have the option of leaving to a higher paid job if thats what you want. Its pretty simple really!
EDIT: Sorry I read that you are "outside london", but really you said you are "just outside london" - does that mean you can commute into london? If so then at 23k you are being paid far below market rates.
Basically I'm a sole developer for this company - I do everything from developing the Rails app itself to managing a bunch of servers running Docker / Docker Swarm. Fun.
If I were I would start applying to jobs, and when they ask you what salary you would expect I would go for 35k, you could reasonably push it to 40k depending on how confident you are at negotiating.
Feel free to PM me if you have anymore questions.
45 - 50 ( with bonus ),
60 - 80 ( with bonus )
The JS field is booming if you are passionate there are many desperate companies I think I may even be getting underpaid in my new role But I have WFH and perks :)
Also I've had 3 months notice on both my jobs it sucks but many companies will wait ( especially larger ones ). Although my new one is 1 month luckily
Do your research and you will see what the going rate is in your location.
You are in a good position to get that pay rise, because your current employer would have to backfill and pay market rates anyway.
In terms of your notice period - if you don't like working there anymore, quit now and then look for jobs after month 1 or 2, so that would give mean your new employer would have to wait only a month.
Or consider contracting.