Ongoing situation at "Big Sister's" house...

I asked in here a few months ago about various problems with Big Sis's
plumbing, however, time moved on and her leaking plumbing disappeared
and she set her sights on getting the whole plumbing replaced when she
could find a plumber prepared to take on the job...

Then, yesterday, her leak came back with a vengance.

She apppears to have an indirectly heated hot water system (standard
cylinder, 3-port valve, boiler downstairs, water pump etc), with a feed
and expansion tank installed a couple of foot above the cylinder, all
contained within an airing cupboard type of arrangement.

The leak appears to be coming somewhere from the Feed and Expansion
tank. Unfortunately, this sits on a piece of delaminating plywood which
acts as a sponge, so wherever the leak is, it drips into the plywood
which distributes is over the complete area of the airing cupboard
making tracing the leak very difficult. Coupled to the fact that the F&E
tank size is pretty much the internal dimensions of the airing cupboard
makes the whole thing a right bugger to work on.

Last night, having fused the CH circuit from water dripping onto the
electrics (sigh!), I went around to take a closer look. Having removed
the architrave surround to get a glimpse of the feed (and overflow)
pipework going to the F&E tank, I was somewhat alarmed to see that the
side of the tank that takes the feed/overflow buldged to an extent that
there was a horizontal deflection of some 4-5 cm halfway down the side
of a 50cm tall tank.

To me it looked wrong, and could indicate where the tank has failed / is
leaking. but until I can get there as a respectable plumbing time (i.e.
not 9pm, and when I've charged my torch [hate it when they die after 2
mins use], and when I've got a set of step-ladders with me) I couldn't
investigate further. In fact, I'm of the mind to replace the tank anyway.

So, to a question or two...

Is it commonplace for F&E tanks to incorporate such a large "buldge" in
the design. Is it the considered opinion of those that deal with such
stuff that this is likely to represent the point of failure of the tank?

How much would a replacement tank cost (existing tank approx 80cm wide,
50cm deep, 50 cm high)? (I've tried online but there're few hits on F&E
tanks, closest I got was loft tanks in Screwfix) [actually, cold light
of day, their "10500" 25gal tank looks ok - is a loft tank the same as a
F&E tank?)