I did get a reply from the 3 executive office.
My account has been credited with £10 to make up for the costs incurred trying to sort out the top up, which I'm satisfied with.
They say that there were no known issues with the web service but it could have been that their IT systems were being updated whilst I was trying to top up. I can understand a few minutes (or maybe even an hour) of downtime - but the fact is I couldn't use the facility for half a day. My feedback has been passed along to the website management team so maybe they'll uncover something, or have incorporated fixes into the new design. I'm still a little concerned that the TopUp web server is overloaded or has issues, because it's always loaded at dial-up speeds and still does now, even after the revamp. But we'll see how well it works in the future.
I did see the option to combine a credit/debit card payment with my £10 account credit to purchase a 3GB (£15) top up, but I was hesitant to use this facility as I knew that previously it took a matter of days to register a card - I had assumed this still applied on the new system and so I missed out on getting more allowance. I definitely would have spent the extra £5 to gain 2GB more allowance had I known.
The other feedback I got was regarding the customer support telephone number. They suggested that if I had a 3 handset I should call 333 or 500 free of charge, but with a PAYG Mobile Broadband SIM card I cannot do this. I do not have a 3G compatible mobile phone to even try it. So I was forced into calling their 0843 number from a mobile phone on Orange and on O2. They should consider the fact that a lot of people with mobile broadband will be using it because they have no landline phone! Customers in this position face potentially HUGE phone bills! But I suspect 3 are not the only company making bundles of money from calls to customer service or technical support.
I was advised about the lack of a customer support email address and was told about their web page based email form. They didn't acknowledge my complaint that I had already tried to submit an email via this system, only to be presented with an error!
So that's about it really. Happy to have been credited £10, but I found the explanations a bit sketchy to be honest. I'll carry on using the service and see how the TopUp system fares next time.