Any business that has contact with the public, whether renting out inflatables or selling groceries, needs to have public liability insurance. If a claim is made and you are found liable it could cause serious financial problems for you as well as for your business.
If you employ anyone outside of family or even have volunteers working for you, whether full time or part time you will require employers' liability. Unlike public liability, employers' liability is a legal requirement in the UK.
If you cannot afford to replace any or all of your equipment then you may need to consider obtaining some equipment cover. It is always worth speaking to an insurance broker when arranging public liability as when purchased together it may work out more cost effective than arranging a mid-term adjustment to your policy.
These are insurance covers that most start-ups should consider. There are of course many other optional add-ons that different insurers either offer or may already include in your policy so it's always worthwhile speaking to a broker before using an online quote and buy. Most importantly, however you arrange your insurance always check the exclusions and that the level of cover is appropriate.
The average cost of a policy for a small bouncy castle business start-up is around £300 but of course, this does depend on a number of factors, such as equipment value etc.