Hello Dan,
Sorry to read of your problem, and I can understand how you may feel you have been let down.
Servicing is usually carried out to a prescribed list of items. However I would expect you to be advised of any glaring problems that stand out, but technicians would not have time to make close inspections of all fittings within a normal service programme. So if you want a close inspection you must first of all specify it, and be prepared for your service and inspection to cost quite a lot more.
Just thinking about it, if the cracks have only just become apparent, How could the dealer possibly have warned you about them earlier? If the cracks have been there during preceding services, why didn't you tell the dealer about them then?
You should always advise your dealer as soon as possible of any faults of this nature. This maximises your chances of getting repairs carried out under warranty.
On the face of it I don't think the dealer has negligent on this matter.
You should read you warranty documents carefully to see if the shower tray was covered, and if so for how long, but I'll bet Bailey are probably right about it being out of warranty, well at least out of Bailey's Manufactures warranty, but that is not necessarily the whole story.
In your post you use a phrase "....is this fit for purpose?" so I suspect you are aware of the Sale of Goods Acts (SoGA) which gives you some quite powerful statutory rights. Manufactures warranties can only add to your statutory rights, they can never override them. But given the circumstances you outline in your post, SoGA is not necessarily going to be easy to use, partly because it seeks to be fair and reasonable offering no advantage or disadvantage to the customer over another customer with a fault free product.
In a nutshell, SoGA automatically comes into play as soon as you make retail purchase, and it requires the sellers to sell goods of satisfactory quality.
Don't forget that even though the tray appears damaged, it has not stopped you using the caravan, and you have had value from it so it has not been a total loss. I suggest you considerately approach your seller and point out that its unreasonable for a shower tray to fail in this way in such a relatively short time. In the light of SoGA, would they consider replacing or repairing it, even just contributing to the repair. That might work.
However if they flatly refuse then you may have to consider using the small claims court and SoGA - Its not going to be easy, because the matter hinges in how the cracks were caused, is it due to an inherent design or manufacturing procedure or weakness, or has there been any opportunity for the tray to be damaged or used inappropriately precipitating the damage since the purchase.
The onus is on you to be able to prove that on the balance of probabilities, the fault was inherent in the design or workmanship, and not caused by inappropriate use or even the wrong cleaning materials. Bearing in mind the length of time you have had the caravan, the chance that something has damaged the tray is considerable, and proving otherwise is going to be an uphill struggle.
The choice is yours, but based on the information so far provided I seriously question whether the hassle, time and cost is worth the effort, when I guess there is less than a 20% chance of winning.