Unfortunately was not able to get Alfie's equipment working, but was able to isolate it as a speed controller problem. Resoldering a few connectors didn't help so it looks like it is an electronic problem which is then beyond my skill and my eyesight.
A few suggestions for replacement ESCs:
Blue series 40A
This is the controller I had attached to my foamy model. I've got a couple of these and have been using them without problems for a couple of years. Unfortunately they are out of stock so you could be waiting a while for delivery.
Blue series 50A
In stock...a few dollars more than the 40A controller. I'd assume it would work as reliably as the 40A ones but have not had one myself.
Turnigy Plush 40A
I've not used these but they do get good reviews.
YEP 45A
I've not used these but they are a copy of the very expensive German YGE controllers and as such get good reviews.
I looked at the Giant Shark website but their decent controllers (Hobbywing) are out of stock. If you want to get a controller by next weekend you could try one of these from Robotbirds. I haven't used them but the company is well respected so you should be OK.
Robotbirds pro 40A
Robotbirds basic 40A
Mike