I wish I had seen all these reviews before shelling out the money for this 'experience'.
They are without a doubt the worst company I have ever dealt with.
The weather in the UK isn't great so its reasonable and understandable that many jump days are out of their control. However I'm sure many times they can quite easily guess there's going to be a 90% chance that people won't be jumping and should really give people a warning since people travel from all over the UK to get there, only to find out they have wasted their time.
As for turning up at 8:00 to be one of the first jumpers as they suggest, you'll actually want to be looking at about 7am if you want to make a jump on the day. They try to cram so many people into each day that many are often sent home.
After trying 4 or 5 times you will loose patience with the company, but don't expect a refund. There staff are well seasoned at answering questions about refunds and will quite frankly tell you to just sod off and try another day.
If you want to jump then try somewhere else because there's a high chance that you'll spend many trips there before you finally get a chance.
