My back problems prevent me from doing normal tasks without assistance
Describe the assistance you need on the form and provide medical evidence.
my asthma causes me to be short of breath which affects how I am able to mobilise, cannot walk too far without being severely short of breath
Do you have medical evidence that your asthma is not controlled by medication?
e.g. How many times do you have to have a meds review, attend the GP/asthma Nurse /clinic/hospital to have dosages changed?
Get the documentation from your GP records, you may have to pay.
Do you have a Blue Badge?
How far do you think you could walk without difficulty? How far do you consider to be "too far" ?
One length of a supermarket aisle (not counting walking there from the car park) is not enough to score you enough points for the mobility section.
and palpitations/SVT is an issue which affects how I move around and complete certain tasks,
This is the sort of thing that you need to put on the PIP form.
You need to explain all of this on the form in detail and provide medical evidence to confirm it.
cardiology have advised I have an operation
Again, provide medical evidence, but I hate to say this, be prepared to hear that your current condition is only temporary, so may not be considered, PIP is for permanent disability.
PIP is awarded on how your conditions affect your daily living.
You do need to read the guides, they explain all the questions in detail, but you also need to "convince" the DWP that you cannot do some tasks reliably, repeatedly and in a timely manner (from memory I think that's more than two and a half times longer than a person without difficulties to even be considered.)
It is a horrible system to go through, you must persevere and take it to Tribunal if necessary - and it often is.