Short answer
Spend nothing until the free things are finished, because a complete Google Business Profile and a review habit cost only time and outperform most paid work. Then put the first real money into tracking and a small local ad budget, in that order.
Most advice on this question is useless because it refuses to be specific. Here is what I would actually do with five hundred pounds, in order, and the part of it I would spend first is nothing.
Before you spend anything
Two jobs, both free, both of which will outperform most things you could buy.
Finish your Google Business Profile. Categories, service areas, services list, real photos, hours that match how you actually work. This is the single highest return work available to a plumbing business and it costs a couple of hours. Most profiles I open have never been completed.
Start asking for reviews. Properly, every job, with a direct link. Not when you remember. Set up something automatic even if it is a text shortcut on your phone.
If you do nothing else, these two will move more work than five hundred pounds spent badly. Do them first, genuinely, before reading on.
Then the first real money
Tracking, about fifty pounds a month. Call tracking so you can see which calls came from where.
This feels like the least exciting purchase on the list and it is the most important, because every decision after it depends on knowing what worked. Without it you will be judging your marketing on whether last week felt busy, which is how people end up spending years on things that never worked.
A small ad budget, the rest of it. Run Google Ads tightly: your best two or three services, a radius you will genuinely drive, and the hours somebody actually answers the phone.
Four hundred pounds will not win a city. It will tell you what a click costs in your town, which searches turn into jobs, and whether paid search works for you at all. That information is worth more than the handful of jobs it brings.
What I would not spend it on
A new website. Not at this budget. A cheap site built badly is worse than the dated one you already have, and you will pay twice when you replace it. If your current site loads on a phone and has a tappable number, it will do for now.
Printed anything. Flyers, magazine adverts, sponsoring the local team. These can work, but you cannot measure them, which means you will never know whether to do it again.
An SEO package from a cold email. The ones offering first page rankings for a hundred a month are selling automated rubbish. Real local SEO costs more than that because it is a person doing work.
Anything with a twelve month contract. At this stage you are buying information, not committing to a strategy.
What good looks like after three months
You should be able to answer: how many calls did I get, where did they come from, and what did each booked job cost me.
If you can, the five hundred did its job even if the jobs it brought barely covered it, because you now know which direction to put the next five hundred. If you cannot answer that, the money went on the wrong things regardless of how busy you felt.
The thing most plumbers get wrong
Spreading it thin. A bit of ads, a bit of social, a cheap directory listing, a flyer drop. Five things at a hundred pounds each, none given enough to work, none measured.
One thing done properly, measured honestly, beats five things done cheaply every time. That is as true at five hundred pounds as it is at five thousand.