This effectively comes under duty of care. Firstly, in England and Wales there is no obligation to be a Good Samaritan - in other words, there is no obligation to be a rescuer. Until you intervene to try and rescue someone, you do not owe that person a duty.
As soon as you do intervene, however, you do owe them a duty. Specifically, you owe them a duty not to make the situation worse (Horsey and Rackley, Tort Law, 3rd ed., OUP 2013, p. 75). The specific situation Horsey and Rackley give is that of resuscitating a drowning child and breaking a rib as you do so: this may be 'making the situation worse' (Horsey and Rackley, pp. 75-76).
Does this mean that you'll be liable if you give someone first aid and in doing so, you make the situation worse? Not necessarily, because, as Horsey and Rackley point out, duty is different to liability. Using the drowning child example again, they state:
So, for example, while someone who intervenes may owe a duty not to make the situation worse, their actions would still be judged against those of a 'reasonable person' in the circumstances (and so if a reasonable person would have tried to resuscitate the child in the same way, there will be no breach of their duty and therefore no liability to pay compensation.) (p.76)
The 'reasonable person' standard corresponds to what you mentioned in the question about lack of expertise. If a doctor intervenes in such a situation, the standard of care they'd be expected to give would be higher than, say, for someone who's simply done a basic first aid course. The question is whether or not you've acted as the reasonable person in your situation would have done.
On that basis, then, your tutor is pretty much correct: so long as you take such care as is reasonable based on your expertise, or lack thereof, then under English and Welsh law, you're unlikely to be liable.