I don't want to be rude, but I think you're going to have a hard time attracting any developers with an £18k salary. Are there any perks people should be aware of to make up for the low pay?
This website [0] gives the 10th percentile for Graduate Web Developers UK-wide as £21,250 putting you firmly at the bottom end of the scale.
[0] https://www.itjobswatch.co.uk/jobs/uk/graduate%20web%20devel...