Hi all,
Thought I would update with where I am at (for anyone who is in the same boat).
I have to have a Dell for work, so currently whittled down to the following:
XPS 15 9560
Inspiron 15 7000 (Gaming)
PassMark - CPU Mark			
Single Thread Performance - Updated 7th of February 2017			
Processor	CPU Mark	Price (USD)	Model
Intel Core i7-7820HQ @ 2.90GHz	2,252	NA	Precision M5520 & M3520
Intel Core i7-6920HQ @ 2.90GHz	2,053	NA	Precision M7710 & M7510 & M3520
Intel Core i7-7700HQ @ 2.80GHz	2,021	NA	Precision M7510, Inspiron 15 7000, XPS 15 9560Intel Core i5-6440HQ @ 2.60GHz	1,898	NA	Precision M3510
Intel Core i7-6820HQ @ 2.70GHz	1,895	NA	Precision M7710 & M7510 & M5510 & M3510
Intel Core i5-7300HQ @ 2.50GHz	1,871	NA	Precision M5520 & M3520
Intel Core i5-7300HQ @ 2.50GHz	1,876	NA	Precision M5520
Intel Core i7-6700HQ @ 2.60GHz	1,801	$1,509.00*	Precision M5510 & M3510
Intel Core i5-6300HQ @ 2.30GHz	1,671	NA	Precision M3510 & M5510 & M7510
Both the same processor (note we looked at Precision but spec is not ideal for Sketchup)..
PassMark - G3D Mark			
High End Videocards - Updated 9th of February 2017			
Videocard	G3D Mark	Price (USD)	Model
**GeForce GTX 1050 Ti	5,888	$139.99	Inspiron 15 7000 (4GB)**Quadro M5000M	5,097	NA	Precision M7710
**GeForce GTX 1050	5,076	$104.99	XPS 15 9560 (4GB)*Quadro M3000M	4,032	NA	Precision M7710
Quadro M4000M	3,627	NA	Precision M7710
Quadro M1200	3,524	NA	Precision M5520
Quadro M620	3,369	NA	Precision M3520
Quadro M2000M	2,550	NA	Precision M7510
Quadro M1000M	1,963	$200.89	Precision M7510 & M5510
FirePro W5130M	1,807	NA	Precision M3510
So the Inspiron has a slightly better spec graphics card but the XPS has a nicer package (read issues with the Inspiron screen).
However, to chuck spanner in the works, we have just been offered the following deal:
Alienware 15 - R3
• Processor: Intel
 Core
 i7-6820HK (4 Core, Dynamically Overclocked up to 4.1GHz,8M Cache, 45W)
• Windows 10 Home (64bit)
• 16GB (2x8GB) 2667MHz DDR4 Memory Non-ECC
• 1 TB SATA Hard Drive (7200 RPM)
• 256GB PCIe M.2 SATA3 Solid State Drive
• No Optical Device
• NVIDIA GTX 1070 8GB
• Software
• Killer 1435 802.11ac 2x2 WiFi and Bluetooth 4.1
• LCD Cover Assembly
• Power Cord : 250V
• Primary 6-cell 99W/HR Battery
• 15.6 inch UHD (3840 x 2160) IGZO IPS Anti-Glare 300-nits Display
• Office 2016 H&B
£1650 + VAT
Although the CPU not as good (though it does have more cache), rest of spec is excellent and looks like a good deal to me..