Thank you all for your suggestions, I will be delving in deep into this issue this weekend (or when I find the time) and I'll get back to you all with updates.

And thank you all for being very helpful, especially to accommodate a newbie!

The fuel pickup tube may actually be the cause because I know that the pump has been fiddled with due to the dust cover being all funky and installed improperly - that will be where I will start.

As to there being multiple problems at once... I'm almost certain this has something to do with it, but I'll be starting with a solid fuel pump and line inspection.