There have been some excellent replies here, most of which are very relevant to me and my situation.
First of all I absolutely agree that my planning, map and PLOG skills should be honed to the best of my ability and that this will only come with more experience and will be something I shall keep doing throughout my flying life.
However the idea of a GPS unit gives me a comforting sense of back-up to my primary navigation skills. I know that it is very easy to get 'behind' the aircraft very quickly due to a situation for example, where you are navigating, talking on the radio, then whoops just gained 300ft, traffic to your 2 o'clock, getting close to that Class E airspace boundary, can't go higher than 3,000ft due to cloud, keeping eyes peeled for a safe landing place if it all goes wrong, destination airfield has just changed from 08L to 19R so how does that affect my overhead join and land...
Good pre-flight planning is of course extremely important and the key is that none of the things I’ve learnt during my training and over that past 15 hours will change if i have a GPS. I will not become lazy and forget about pre-flight planning, I will not be relying solely on my GPS (if at all), but it will be there if i need it and a quick glance to reassure me is worth it's weight in gold when the going gets tough.
This kind of reminds me of when I first took up scuba diving, initially going to 30m was a daunting prospect (especially in the North Sea), so I decided to buy a pony bottle (a small spare tank with it's own reg) this gave me a tremendous sense of confidence (not to be confused with over-confidence - I am aware of my limitations) and helped me have some truly excellent dives. Fortunately I never needed to use it and now I’ve got over 3,500 dives i feel my skills and my awareness are such that i can handle most situations so I don’t take it with me anymore. However it was very reassuring knowing I could use it when I was new to the sport and gave me one less thing to worry about.
Hopefully, I will be able to achieve a similar amount of pleasure in my flying career, but these initial few hundred hours are the ones where any backup will stand me in good stead. After all, it would be better for me to have it and not use it, than to wish I had it when I can feel it going pear shaped...