Programming nRF52 with OpenOCD

Hi, I have a BMD-300 (nRF52) module that I'm attempting to program with OpenOCD and an ST-link from Linux (it worked great for nRF51 so I was trying to keep the same tools).

$OPENOCD/src/openocd -s $OPENOCD/tcl -f interface/stlink-v2.cfg -f target/nrf52.cfg -c init -c "reset init" -c halt -c "nrf52 mass_erase" -c "program $FIRMWARE verify" -c reset -c exit

I'm using OpenOCD from here: github.com/.../openocd

And it says:

Open On-Chip Debugger 0.10.0-dev-00282-gbd6642f (2016-05-09-13:36)
Licensed under GNU GPL v2
For bug reports, read
	openocd.org/.../bugs.html
Info : auto-selecting first available session transport "hla_swd". To override use 'transport select <transport>'.
Info : The selected transport took over low-level target control. The results might differ compared to plain JTAG/SWD
adapter speed: 10000 kHz
Info : Unable to match requested speed 10000 kHz, using 4000 kHz
Info : Unable to match requested speed 10000 kHz, using 4000 kHz
Info : clock speed 4000 kHz
Info : STLINK v2 JTAG v14 API v2 SWIM v0 VID 0x0483 PID 0x3748
Info : using stlink api v2
Info : Target voltage: 3.787037
Info : nrf52.cpu: hardware has 0 breakpoints, 2 watchpoints
Error: timed out while waiting for target halted
TARGET: nrf52.cpu - Not halted
in procedure 'reset' 
in procedure 'ocd_bouncer'

It definitely knows the chip is connected and can read the chip ID off it, as it comes up with different errors with the chip connected and trying to flash with the nrf51 code returns:

Warn : UNEXPECTED idcode: 0x2ba01477
Error: expected 1 of 1: 0x0bb11477

Do you have any ideas what the problem might be? Error: timed out while waiting for target halted seems like the issue, but I'm not sure how to fix it... As I say, this worked (connecting SWCLK, SWDIO, GND and using target power) on nRF51 just fine. Could it be to do with the BMD-300's existing bootloader?