42k/month is not good or bad. If it is good or bad depends on the person (job+industry+nationality) and how much experience & qualifications this person has. I know many people who earn more than this with 10 years experience and many people who earn less than this but have 20 years experience. 42k/month however is plenty enough to live a very decent life here in Doha and still save enough. The situation here however is that those who come out for the first time generally find it hard to get the best package and with the NOC system in place it is not possible to move to another employer within the country after moving here (unless your employer gives an NOC and the majority do not). So do your research and speak to a contact within the industry to find out what the current going rate is. My rule of thumb would be to ask for a 20% uplift to your current UK salary and private medical. If can get flights, accommodation, car ..... in addition then that is even better but 20% uplift to the current UK salary is a must. Another thing to note is the traffic and contracted hours. I used to do 37.5hr/week in the UK but am doing 48hrs/week here. So an increase in working hours (contracted working hours mind you not the overtime hours that we all put in) of over 20% easily justifies the extra money that you ask for. Another point to note is that working hours may be even longer at some places. For instance I know of construction sites where the contractor's staff are required to do 6day weeks with 10hours per day (so 60hour weeks - in which case even a 50% increase to current UK salary is justifiable). The tax free is not an employer offered benefit. That is a state benefit as Qatar is rich enough to not require people who work hard to pay for the privilage of working. So dont get suckered with tax free as a reason to work harder and longer hours.
Accommodation here is higher than London prices in places. The nicer apartments are quite expensive. QAR 10k/month for a 1 bed apartment is considered cheap in some buildings. If you do decide not to splash out it is possible to rent a 2 bed flat for less than QAR6,000/month (I recently went to view one which was being vacated by a Brit going back home after about 10 years in the middle east) so there is nothing to say that you could not find cheap accommodation. Some people pay even less but live further away (check for Ezdan properties in Wakra for instance). So 42k/month is certainly enough to have a really good life here but if that is the correct salary would depend on a large number of factors.