This is quite a hungry tomato which produces large fruit, so expecting it do well in a pot by a window is being very optimistic, especially if the pot is less than around 14 inches. I've never heard the 'subject to infestation' point as these are grown predominantly as outdoor tomatoes and in fact they often abort in the high temperatures of a greenhouse.
There are many more much better large tomato varieties which I'd recommend you try next year/

Thanks for coming to my rescue once again! This was a gift - not a variety I'd have chosen. Is it too late to repot (currently in approx 12") and put outside? I have no greenhouse so would be on south-facing patio. If so, how should I protect it?

It would probably benefit from being given more space, even this late, but it would be probably better to give it some drip irrigation of tomato feed and water outside. A cheap way of doing this would be by making a pin hole in an old plastic water bottle that can drip on to the plant. Keep the water and feed coming and I'm sure it will do much better. It shouldn't need any protection outside in July esp. on a south facing patio, though I'd check it if the weather turns wet and humid in case of blight. A temporary polythene cover would help protect it.
Good luck with your crop.

Thanks once again - I'll give it a go! Do I keep the cover on at all times or just when it's damp and/or humid? By 'infestation' I meant blight - sorry - as I said, am new to this!