If it's only just started happening, don't worry about it too much and just change it soon.
The Laser tool is £10-£15
Pulley is £50+vat ish (receipt is in the car)
Belt was £30+vat ish
You only need the Laser 3307 tool, a 19mm spanner (ratchet spanner would be best imo), and a wrench to hold the Laser tool and about 15-30mins to change the belt and pulley.
I can get an image of the belt layout over the various pulleys if you want for when you refit the belt.